fs: btrfs: fix false negatives in ROOT_ITEM search

ROOT_ITEMs in btrfs are referenced without knowing their actual "offset"
value. To perform these searches using only two items from the key, the
btrfs driver uses a special "btrfs_search_tree_key_type" function.

The algorithm used by that function to transform a 3-tuple search into a
2-tuple search was subtly broken, leading to items not being found if
they were the first in their tree node.

This commit fixes btrfs_search_tree_key_type to properly behave in these
situations.

Signed-off-by: Pierre Bourdon <delroth@gmail.com>
Cc: Marek Behun <marek.behun@nic.cz>