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/109

@@ -63,7 +63,7 @@ populate()
 	done
 
 	echo "flushing changes via umount/mount."
-	umount $SCRATCH_DEV
+	_scratch_unmount
 	_scratch_mount
 }
 
@@ -91,7 +91,7 @@ allocate()
 }
 
 # real QA test starts here
-umount $SCRATCH_DEV 2>/dev/null
+_scratch_unmount 2>/dev/null
 _scratch_mount
 
 # see if faststart is possible (and requested)
@@ -100,7 +100,7 @@ faststart=""
 if [ -n "$FASTSTART" -a -f $SCRATCH_MNT/f0 ]; then
 	faststart="-N"	# causes us to skip the mkfs step
 fi
-umount $SCRATCH_DEV
+_scratch_unmount
 
 _scratch_mkfs_xfs -dsize=160m,agcount=4 $faststart | _filter_mkfs 2>$tmp.mkfs
 cat $tmp.mkfs >>$seqres.full