glusterd/snapshot: Use external umount for un-mounting snapshot

umount2 doesn't cleanup /etc/mtab entry after doing umount,
so use external umount command for un-mount operation.

Change-Id: I1a91a700433e2bf15dd21e6fcdd9da54441048d1
BUG: 1098084
Signed-off-by: Vijaikumar M <vmallika@redhat.com>
Reviewed-on: http://review.gluster.org/7775
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>