btrfs-progs: find-root: fix a bug that will cause wrong result

btrfs-progs: find-root: fix a bug that will cause wrong result

[BUG]
btrfs-find-root may not output desire result, as due to
search_extent_cache() may return a result that doesn't cover the desired
range, generation cache can be screwed up if higher generation tree root
is found before lower generation tree root.

For example:
=======
./btrfs-find-root  /dev/sda6  -a
Superblock thinks the generation is 8
Superblock thinks the level is 0
adding bytenr: 4194304, gen: 8	<<< Debug output
adding bytenr: 24715264, gen: 7 <<< gen is 7 at read_tree_block time
Well block 4194304(gen: 8 level: 0) seems good, and it matches
superblock
Well block 24715264(gen: 8 level: 0) seems good, and it matches
superblock  <<< But its gen is wrong at result output time
=======

[Fix]
Add a new check to make sure the search_extent_cache() is returning the
desired result.

Reported-by: Marc Merlin <marc@merlins.org>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>