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/244

@@ -39,7 +39,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 _cleanup()
 {
 	cd /
-	umount $SCRATCH_MNT 2>/dev/null
+	_scratch_unmount 2>/dev/null
 	rm -f $tmp.*
 }
 
@@ -96,7 +96,7 @@ if $XFS_QUOTA_PROG -x -c "project -s -p $dir/over32bit 5344967296" $SCRATCH_DEV
 fi
 
 #  Do testing on filesystem with projid32bit feature enabled
-umount $SCRATCH_DEV 2>/dev/null
+_scratch_unmount 2>/dev/null
 _scratch_mkfs_xfs -i projid32bit=1 >> $seqres.full || _fail "mkfs failed"
 _qmount
 mkdir $dir