tests: Fixed spurious failure in bug-887098-gmount-crash.t

This script was trying to see if the mount process died by doing a
'ps ax' and a grep of the original pid in the results. After that
the pid of the first line returned by grep was compared to the
original pid.

This method can lead to false negatives because it's possible that
the original pid appears in some other part of the 'ps ax' list.

This patch uses get_mount_process_pid() from volume.rc to check if
the process is still alive.

Change-Id: I0285366e601a146793c47e9c1156a4bb36d6fcb3
BUG: 1092850
Signed-off-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-on: http://review.gluster.org/8286
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>