signal: In get_signal test for signal_group_exit every time through the loop

Recently while investigating a problem with rr and signals I noticed
that siglock is dropped in ptrace_signal and get_signal does not jump
to relock.

Looking farther to see if the problem is anywhere else I see that
do_signal_stop also returns if signal_group_exit is true.  I believe
that test can now never be true, but it is a bit hard to trace
through and be certain.

Testing signal_group_exit is not expensive, so move the test for
signal_group_exit into the for loop inside of get_signal to ensure
the test is never skipped improperly.

This has been a potential problem since I added the test for
signal_group_exit was added.

Fixes: 35634ffa ("signal: Always notice exiting tasks")
Reviewed-by: NKees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/875yssekcd.fsf_-_@email.froward.int.ebiederm.orgSigned-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

kernel/signal.c

@@ -2662,6 +2662,9 @@ bool get_signal(struct ksignal *ksig)
 		goto relock;
 	}
 
+	for (;;) {
+		struct k_sigaction *ka;
+
 		/* Has this task already been marked for death? */
 		if (signal_group_exit(signal)) {
 			ksig->info.si_signo = signr = SIGKILL;
@@ -2672,9 +2675,6 @@ bool get_signal(struct ksignal *ksig)
 			goto fatal;
 		}
 
-	for (;;) {
-		struct k_sigaction *ka;
-
 		if (unlikely(current->jobctl & JOBCTL_STOP_PENDING) &&
 		    do_signal_stop(0))
 			goto relock;