io_uring/cmd: consolidate REQ_F_BUFFER_SELECT checks

io_uring/cmd: consolidate REQ_F_BUFFER_SELECT checks

io_uring_cmd_prep() checks that REQ_F_BUFFER_SELECT is set in the
io_kiocb's flags iff IORING_URING_CMD_MULTISHOT is set in the SQE's
uring_cmd_flags. Consolidate the IORING_URING_CMD_MULTISHOT and
!IORING_URING_CMD_MULTISHOT branches into a single check that the
IORING_URING_CMD_MULTISHOT flag matches the REQ_F_BUFFER_SELECT flag.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250821163308.977915-4-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>