io_uring/memmap: drop unused sz param in io_uring_validate_mmap_request()

io_uring_validate_mmap_request() doesn't use its size_t sz argument, so
remove it.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>