io_uring: rely solely on FMODE_NOWAIT

Now that we have both sockets and block devices setting FMODE_NOWAIT
appropriately, we can get rid of all the odd special casing in
__io_file_supports_nowait() and rely soley on FMODE_NOWAIT and
O_NONBLOCK rather than special case sockets and (in particular) bdevs.

Link: https://lore.kernel.org/r/20230509151910.183637-4-axboe@kernel.dk


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