io_uring: io_async_buf_func() need not test page bit

Since we don't do exclusive waits or wakeups, we know that the bit is
always going to be set. Kill the test. Also see commit:

2a9127fc ("mm: rewrite wait_on_page_bit_common() logic")

Signed-off-by: Jens Axboe <axboe@kernel.dk>