nbd: set sk->sk_sndtimeo for our sockets

If the nbd server stops receiving packets altogether we will get stuck
waiting for them to receive indefinitely as the tcp buffer will never
empty, which looks like a deadlock.  Fix this by setting the sk send
timeout to our configured timeout, that way if the server really
misbehaves we'll disconnect cleanly instead of waiting forever.

Reported-by: Dan Melnic <dmm@fb.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>