fstests: filter test and scratch together safely

Several tests uses both _filter_test_dir and _filter_scratch
concatenated by pipe to filter $TEST_DIR and $SCRATCH_MNT. However,
this would fail if the shorter string is a substring of the other
(like "/mnt" and "/mnt2").

This patch introduces new common filter function to safely call both
_filter_test_dir and _filter_scratch, and update tests and functions
to use this new function.

I checked this with btrfs/029, generic/409,410,411, and
generic/381,383, xfs/106,108 (which calls _filter_quota). Thanks
Eryu for advice.

[eguan: folded 2nd patch into 1st patch and update commit log a bit]

Signed-off-by: Tomohiro Misono <misono.tomohiro@jp.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

tests/btrfs/029

@@ -66,19 +66,14 @@ _scratch_mkfs > /dev/null 2>&1
 _scratch_mount
 $XFS_IO_PROG -f -c 'pwrite -S 0x61 0 9000' $SCRATCH_MNT/original >> $seqres.full
 
-_filter_testdirs()
-{
-    _filter_test_dir | _filter_scratch
-}
-
 _create_reflinks()
 {
     # auto reflink, should fall back to non-reflink
     rm -rf $2
     echo "reflink=auto:"
     cp --reflink=auto $1 $2
-    md5sum $1 | _filter_testdirs
-    md5sum $2 | _filter_testdirs
+    md5sum $1 | _filter_testdir_and_scratch
+    md5sum $2 | _filter_testdir_and_scratch
 
     # always reflink, should fail outright
     rm -rf $2
@@ -86,7 +81,7 @@ _create_reflinks()
     cp --reflink=always $1 $2 >> $seqres.full 2>&1 || echo "cp reflink failed"
 
     # The failed target actually gets created by cp:
-    ls $2 | _filter_testdirs
+    ls $2 | _filter_testdir_and_scratch
 }
 
 echo "test reflinks across different devices"