btrfs-progs: convert-tests: Add test case for backup superblock migration

New convert introduced simpler chunk/extent allocation algorithm, at the
cost of complex backup superblock migration codes.

Use specially built ext2 images to test if btrfs-convert can convert and
rollback images without problem.

All these special ext2 image have blocks/holes across 2nd btrfs backup
superblock.

The naming of test image is like the following:
	|<------superblock migration range----->|
	64M					64M + 64K
|-Data--|-Data--|/Hole//|-Data--|/Hole//|-Data--|--Data--| = drdhdhdrd

These test cases should check all typical layouts and make sure new
convert works.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

tests/convert-tests/004-ext2-backup-superblock-ranges/test.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+
+# Specially created e2image dump to test backup superblock migration for
+# new convert.
+# These images will cause the following problems if convert doesn't handle
+# backup superblock migration well:
+# 1) Assert while building free space tree
+# 2) Error copying inodes
+# 3) Discontinuous file extents after convert
+# 4) Overlap file extents
+# 5) Unable to rollback
+
+source $TOP/tests/common
+
+check_prereq btrfs-convert
+check_prereq btrfs
+check_prereq btrfs-show-super
+check_global_prereq e2fsck
+check_global_prereq xzcat
+
+setup_root_helper
+prepare_test_dev 512M
+
+for src in $(find . -iname "*.e2image.raw.xz"); do
+	extracted=$(extract_image "$src")
+	run_check $SUDO_HELPER e2fsck -n -f $extracted
+	run_check $SUDO_HELPER $TOP/btrfs-convert $extracted
+	run_check $SUDO_HELPER $TOP/btrfs check $extracted
+	run_check $SUDO_HELPER $TOP/btrfs-show-super $extracted
+
+	run_check $SUDO_HELPER mount -o loop $extracted $TEST_MNT
+	run_check $SUDO_HELPER e2fsck -n -f $TEST_MNT/ext2_saved/image
+	run_check $SUDO_HELPER umount $TEST_MNT
+
+	run_check $SUDO_HELPER $TOP/btrfs check $extracted
+	run_check $SUDO_HELPER $TOP/btrfs-convert -r $extracted
+	run_check $SUDO_HELPER e2fsck -n -f $extracted
+	rm -f $extracted
+done