net: make socket read/write_iter() honor IOCB_NOWAIT

[ Upstream commit ebfcd8955c0b52eb793bcbc9e71140e3d0cdb228 ]

The socket read/write helpers only look at the file O_NONBLOCK. not
the iocb IOCB_NOWAIT flag. This breaks users like preadv2/pwritev2
and io_uring that rely on not having the file itself marked nonblocking,
but rather the iocb itself.

Cc: netdev@vger.kernel.org
Acked-by: David Miller <davem@davemloft.net>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>