signals: check_kill_permission: remove tasklist_lock

Now that task_session() can't return a false NULL, check_kill_permission()
doesn't need tasklist_lock.

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

kernel/signal.c

@@ -551,9 +551,7 @@ static int check_kill_permission(int sig, struct siginfo *info,
 	    !capable(CAP_KILL)) {
 		switch (sig) {
 		case SIGCONT:
-			read_lock(&tasklist_lock);
 			sid = task_session(t);
-			read_unlock(&tasklist_lock);
 			/*
 			 * We don't return the error if sid == NULL. The
 			 * task was unhashed, the caller must notice this.