Btrfs: add regression test for iterating backrefs

Btrfs send/scrub/defrag/qgroup need to walk backrefs,this test
is to make sure iterating backrefs with ulist is working and don't
cause a kernel panic here.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/btrfs/033

@@ -0,0 +1,71 @@
+#!/bin/bash
+# FS QA Test No. btrfs/033
+#
+# Regression test for iterating backrefs
+#
+#-----------------------------------------------------------------------
+# Copyright (c) 2014 Fujitsu.  All Rights Reserved.
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it would be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write the Free Software Foundation,
+# Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+#
+#-----------------------------------------------------------------------
+#
+
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1	# failure is the default!
+
+_cleanup()
+{
+    rm -f $tmp.*
+}
+
+trap "_cleanup ; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+
+# real QA test starts here
+_supported_fs btrfs
+_supported_os Linux
+_require_scratch
+
+_scratch_mkfs > /dev/null 2>&1
+_scratch_mount
+
+touch $SCRATCH_MNT/foo
+
+# get file with fragments by using backwards writes.
+for i in `seq 10240 -1 1`; do
+	$XFS_IO_PROG -f -d -c "pwrite $(($i * 4096)) 4096" \
+		$SCRATCH_MNT/foo > /dev/null
+done
+
+# get some snapshots here to make sure we are really
+# suffering from walking backrefs.
+for i in `seq 0 50`; do
+	$BTRFS_UTIL_PROG subvolume snapshot -r $SCRATCH_MNT \
+		$SCRATCH_MNT/snap_$i >> $seqres.full 2>&1
+done
+
+$BTRFS_UTIL_PROG send -f $SCRATCH_MNT/send_file \
+	$SCRATCH_MNT/snap_1 >> $seqres.full 2>&1
+
+echo "Silence is golden"
+status=0 ; exit

tests/btrfs/033.out

@@ -0,0 +1,2 @@
+QA output created by 033
+Silence is golden

tests/btrfs/group

@@ -35,3 +35,4 @@
 030 auto quick
 031 auto quick
 032 auto quick
+033 auto quick