selftests: ublk: add batch buffer management infrastructure

selftests: ublk: add batch buffer management infrastructure

Add the foundational infrastructure for UBLK_F_BATCH_IO buffer
management including:

- Allocator utility functions for small sized per-thread allocation
- Batch buffer allocation and deallocation functions
- Buffer index management for commit buffers
- Thread state management for batch I/O mode
- Buffer size calculation based on device features

This prepares the groundwork for handling batch I/O commands by
establishing the buffer management layer needed for UBLK_U_IO_PREP_IO_CMDS
and UBLK_U_IO_COMMIT_IO_CMDS operations.

The allocator uses CPU sets for efficient per-thread buffer tracking,
and commit buffers are pre-allocated with 2 buffers per thread to handle
overlapping command operations.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>