Filesystem: fix getting the wrong block device when doing grep

If we have such a filesystem structure mounted in our cluster
/dev/drbd50 /export/prq/upgrade_emspq xfs
/dev/drbd55 /export/prq/upgrade_sharedmsb xfs
/dev/drbd1 /export/devAGN/share xfs
/dev/drbd5 /export/dev/archivesn1agn xfs

When we want to stop the filesystem mounted on
/export/dev/archivesn1agn, we probably will get the wrong entry returned
here and thus will try to stop the wrong target.