signals: change collect_signal() to return void

With the recent changes collect_signal() always returns true.  Change it
to return void and update the single caller.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>