mount.glusterfs: return 32 in case of double mounting

The mount.nfs helper returns 32 for this particular error case. It makes
sense for mount.glusterfs to return the same error value.

Change-Id: I628f4c93bc796bb096e91857195ffd3d296eaae9
BUG: 1031973
Reported-by: Deepak C Shetty <deepakcs@redhat.com>
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/7469
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>