net: set FMODE_NOWAIT for sockets

The socket read/write functions deal with O_NONBLOCK and IOCB_NOWAIT
just fine, so we can flag them as being FMODE_NOWAIT compliant. With
this, we can remove socket special casing in io_uring when checking
if a file type is sane for nonblocking IO, and it's also the defined
way to flag file types as such in the kernel.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
Reviewed-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20230509151910.183637-2-axboe@kernel.dkSigned-off-by: Jens Axboe <axboe@kernel.dk>

net/socket.c

@@ -471,6 +471,7 @@ struct file *sock_alloc_file(struct socket *sock, int flags, const char *dname)
 		return file;
 	}
 
+	file->f_mode |= FMODE_NOWAIT;
 	sock->file = file;
 	file->private_data = sock;
 	stream_open(SOCK_INODE(sock), file);