__kill_pgrp_info: simplify the calculation of return value

No need to calculate/check the "success" variable, we can kill it and update
retval in the main loop unless it is zero.

Link: https://lkml.kernel.org/r/20230823171455.GA12188@redhat.com


Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: David Laight <David.Laight@ACULAB.COM>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>