build/glusterfs.spec.in: %post server doesn't wait for old glusterd

'killall glusterd' needs to wait for the old glusterd to exi
before starting the updated one, otherwise the new process can't
bind to its socket ports

Change-Id: I78af70419d8b1ac878ee9711acdc01b308b0e46f
BUG: 1113959
Signed-off-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>
Reviewed-on: http://review.gluster.org/8190
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Lalatendu Mohanty <lmohanty@redhat.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>