glusterd: ignore failure to stop a stopped service.

        Backport of http://review.gluster.org/6525

kill(2) returns -1 with errno set to ESRCH when the pid of the process
being killed doesn't exist. Failing glusterd_brick_stop on a stopped
brick could result in volume-stop failing, in commit phase.
This fix prevents that from happening.

Change-Id: I00f46fa06e489a671efbb8e4119f545f8ccea329
BUG: 1038051
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/6568
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>