btrfs-progs: convert: Fix a bug that makes convert asserts at scan time
When a ext2 fs filled with a 57M file, it's possible that convert fails
with assert in add_merge_cache_extent().
The problem is that the ext2 used space just takes some of the second
superblock.
And due to a bug in reserving superblock space, it corrupted used space
tree and cause assert.
Fix in by doing better used space merging for case where superblock
range is inside the ext2 used space.
Reported-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>