keyctl_session_to_parent(): use thread_group_empty() to check singlethreadness

No functional changes.

keyctl_session_to_parent() is the only user of signal->count which needs
the correct value.  Change it to use thread_group_empty() instead, this
must be strictly equivalent under tasklist, and imho looks better.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

security/keys/keyctl.c

@@ -1269,7 +1269,7 @@ long keyctl_session_to_parent(void)
 		goto not_permitted;
 
 	/* the parent must be single threaded */
-	if (atomic_read(&parent->signal->count) != 1)
+	if (!thread_group_empty(parent))
 		goto not_permitted;
 
 	/* the parent and the child must have different session keyrings or