common: use mount/umount helpers everywhere

Replace every explicit mount/umount of scratch or test devices with
helper functions. This allows the next patch to add in hooks to these
functions in order to set up & tear down overlayfs on every mount/umount

(also adds _test_unmount(), which didn't exist prior)

[Eryu Guan rebased the patch agains latest master and replaced more
mount/umount with helpers]

Signed-off-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eric Sandeen <esandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/xfs/111

@@ -45,7 +45,7 @@ rm -f $seqres.full
 [ "$XFS_FSR_PROG" = "" ] && _notrun "xfs_fsr not found"
 
 # real QA test starts here
-umount $SCRATCH_DEV 2>/dev/null
+_scratch_unmount 2>/dev/null
 MKFS_OPTIONS="-bsize=4096"
 MOUNT_OPTIONS="-o noatime"
 _scratch_mkfs_xfs | _filter_mkfs 2>$tmp.mkfs
@@ -61,7 +61,7 @@ done
 
 # Roughly the middle of the created inodes
 INO=`ls -i $SCRATCH_MNT/512 | awk '{print $1}'`
-umount $SCRATCH_DEV
+_scratch_unmount
 
 # Figure out where that middle inode starts
 BYTE=`$XFS_DB_PROG -c "convert inode $INO byte" $SCRATCH_DEV \
@@ -77,7 +77,7 @@ echo Attempting bulkstat
 $XFS_FSR_PROG $SCRATCH_MNT | _filter_scratch
 
 # Fix up intentional corruption so test can pass
-umount $SCRATCH_DEV
+_scratch_unmount
 $XFS_REPAIR_PROG $SCRATCH_DEV >> $seqres.full 2>&1
 
 status=0