Changes Made:

1. Updated Cargo.toml: Changed libublk dependency to use the main branch from
GitHub instead of crates.io to get the latest version with UBLK_DEV_F_SINGLE_CPU_AFFINITY support.

2. Added --multi-cpus-affinity flag: Added a new command-line option to GenAddArgs
in src/args.rs:69 that allows users to opt-out of single CPU affinity.

3. Applied single CPU affinity by default: Modified the new_ublk_ctrl method in
src/args.rs:206-211 to automatically apply UBLK_DEV_F_SINGLE_CPU_AFFINITY unless
the --multi-cpus-affinity flag is specified.

How it works:
- Default behavior: All ublk devices now use single CPU affinity for improved performance
- Opt-out option: Users can specify --multi-cpus-affinity to use the previous multi-CPU behavior
- Universal application: The feature applies to all target types (loop, null, qcow2, compress, zoned)

Signed-off-by: Ming Lei <tom.leiming@gmail.com>

1. Updated args.rs: Modified zero-copy validation to allow null targets alongside loop targets

2. Enhanced null.rs with zero-copy support:
  - Added handle_queue_tag_async_null_zc() function that uses submit_io_cmd_with_auto_buf_reg()
    with UBLK_AUTO_BUF_REG_FALLBACK flag
  - Added handle_queue_tag_async_null() function for regular async operations
  - Modified q_async_fn() to detect auto buffer registration support and select appropriate handler
  - Added validation requiring --async-await when zero-copy is enabled for null targets

Also add sync zc:
  1. Added q_sync_zc_fn() function:
    - Creates auto buffer registration list for all queue depths
    - Uses UblkQueue::submit_fetch_commands_with_auto_buf_reg() for initialization
    - Uses UblkQueue::complete_io_cmd_with_auto_buf_reg() in the I/O handler
    - Handles the shared data using Rc to satisfy borrow checker requirements
  2. Updated queue handler logic:
    - Added condition to use q_sync_zc_fn() when UBLK_F_AUTO_BUF_REG flag is set and async mode is not enabled
    - Maintains existing behavior for async and regular sync modes
  3. Removed async-await restriction:
    - Zero-copy can now work in both synchronous and asynchronous modes
    - Aligns with the new synchronous zero-copy implementation

Key Features:
- Zero-copy support: Null target now supports --zero-copy flag using UBLK_F_AUTO_BUF_REG
- Automatic buffer registration: Uses ublk_auto_buf_reg with fallback mode
- Validation: Requires --async-await flag when zero-copy is enabled
- Performance optimization: Eliminates buffer copies between kernel and userspace

Signed-off-by: Ming Lei <tom.leiming@gmail.com>

Changes Made to src/loop.rs

1. Added lo_handle_io_cmd_sync_zc() function (src/loop.rs:229-253):
  - Zero-copy version of the synchronous I/O command handler
  - Uses UblkQueue::complete_io_cmd_with_auto_buf_reg() instead of complete_io_cmd()
  - Uses __lo_make_io_sqe_zc() for zero-copy SQE generation instead of __lo_make_io_sqe()

2. Added q_zc_fn() function (src/loop.rs:281-301):
  - Synchronous zero-copy queue function using auto buffer registration
  - Creates auto buffer registration list for all queue depths
  - Uses UblkQueue::submit_fetch_commands_with_auto_buf_reg() for initialization
  - Uses shared Rc pattern to handle borrow checker requirements

3. Updated queue handler logic (src/loop.rs:435-447):
  - Added condition to use q_zc_fn() when UBLK_F_AUTO_BUF_REG flag is set and async mode is not enabled
  - Maintains existing behavior for async (q_a_fn()) and regular sync (q_fn()) modes

4. Removed async-await restriction (src/loop.rs:431-433):
  - Zero-copy can now work in both synchronous and asynchronous modes
  - Loop target now supports --zero-copy flag without requiring --async-await

The implementation follows the same patterns as both the existing loop async zero-copy
code and the null target's synchronous zero-copy implementation, ensuring consistency
across the codebase. The loop target now supports zero-copy mode with --zero-copy flag
in both synchronous and asynchronous modes, providing better performance through
automatic buffer registration.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>