This fixes a procfs crash noticed by Anton Blanchard.

The procfs code can have a reference even to an already exited task, so
it needs to follow special rules accessing p->sig.  The atomic-signals
patch made this bug happen at a much higher frequency, but procfs i
believe was buggy ever since, it potentially used the freed signal
structure - which just did not result in a crash like it does today.

The proper fix is to take the tasklist read-lock in
collect_sigign_sigcatch(), this excludes __exit_sighand() freeing the
signal structure prematurely.