freezer: remove should_send_signal() and update frozen()

should_send_signal() is only used in freezer.c.  Exporting them only
increases chance of abuse.  Open code the two users and remove it.

Update frozen() to return bool.

Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/kernel/freezer.c b/kernel/freezer.c
index f53cd5a..95a1238 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -132,7 +132,7 @@
 		return false;
 	}
 
-	if (should_send_signal(p)) {
+	if (!(p->flags & PF_FREEZER_NOSIG)) {
 		fake_signal_wake_up(p);
 		/*
 		 * fake_signal_wake_up() goes through p's scheduler