Fuzzed image bko-156811-bad-parent-ref-qgroup-verify.raw causes qgroup
to report -ENOMEM.

But the fact is, such image is heavily damaged so there is no valid root
item for the extent tree.

Normal extent tree key in root tree should be (EXTENT_TREE ROOT_ITEM 0),
while in that fuzzed image, we got (EXTENT_TREE EXXTENT_DATA SOME_NUMBER).

It's btrfs_find_last_root() that only checks the objectid, not caring
about the key type leading to such problem.

Fix it by doing extra check on key type.

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