nbd.c: sock_xmit: cleanup signal related code

sock_xmit() re-implements sigprocmask() and dequeue_signal_lock().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>