ublk: handle UBLK_U_IO_PREP_IO_CMDS

ublk: handle UBLK_U_IO_PREP_IO_CMDS

This commit implements the handling of the UBLK_U_IO_PREP_IO_CMDS command,
which allows userspace to prepare a batch of I/O requests.

The core of this change is the `ublk_walk_cmd_buf` function, which iterates
over the elements in the uring_cmd fixed buffer. For each element, it parses
the I/O details, finds the corresponding `ublk_io` structure, and prepares it
for future dispatch.

Add per-io lock for protecting concurrent delivery and committing.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>