btrfs-progs: Check the consistence between the parent node and child node/leaf.

btrfs-progs: Check the consistence between the parent node and child node/leaf.

When btrfs-progs walk down the tree, it does not check whether the child
node/leaf is valid.
In fact, there is some corrupted image whose csum is all valid but
parent node points to a invalid leaf.

In my case, the parent node in fs tree point to a invalid leaf(gen 11),
whose generation(15) and first key(EXTENT_TREE ROOT_ITEM 0) is
completely invalid, and will cause BUG_ON in process_inode_item().

Unfortunately, we are unable to fix when it happens.
So we can only output meaningful error message and avoid the insane
node/leaf, which is still much better than the original BUG_ON().

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