init-ceph: stop returns before daemons are dead

The existence of the pidfile must be checked outside of the loop to send
a signal to the daemon. Otherwise the daemon will remove the pidfile and
stop can return before the process is dead because it only checks
/proc/$pid if the pidfile exists.

http://tracker.ceph.com/issues/10389 Fixes: #10389

Signed-off-by: Loic Dachary <ldachary@redhat.com>