btrfs: simple sparse copy testcase for btrfs

Tests file clone functionality of btrfs ("reflinks"):
   - Reflink a file
   - Reflink the reflinked file
   - Modify the original file
   - Modify the reflinked file

[sandeen: add helpers, make several mostly-cosmetic
 changes to the original testcase]

Signed-off-by: Koen De Wit <koen.de.wit@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

common/rc

@@ -2102,6 +2102,27 @@ _require_ugid_map()
 	fi
 }
 
+_require_cp_reflink()
+{
+       cp --help | grep -q reflink || \
+               _notrun "This test requires a cp with --reflink support."
+}
+
+# Given 2 files, verify that they have the same mapping but different
+# inodes - i.e. an undisturbed reflink
+# Silent if so, make noise if not
+_verify_reflink()
+{
+       # not a hard link or symlink?
+       cmp -s  <(stat -c '%i' $1) <(stat -c '%i' $2) \
+               && echo "$1 and $2 are not reflinks: same inode number"
+
+       # same mapping?
+       diff -u <($XFS_IO_PROG -c "fiemap" $1 | grep -v $1) \
+               <($XFS_IO_PROG -c "fiemap" $2 | grep -v $2) \
+               || echo "$1 and $2 are not reflinks: different extents"
+}
+
 _create_loop_device()
 {
 	file=$1