Btrfs: use correct key when repeating search for extent item

If skinny metadata is enabled and our first tree search fails to find a
skinny extent item, we may repeat a tree search for a "fat" extent item
(if the previous item in the leaf is not the "fat" extent we're looking
for). However we were not setting the new key's objectid to the right
value, as we previously used the same key variable to peek at the previous
item in the leaf, which has a different objectid. So just set the right
objectid to avoid modifying/deleting a wrong item if we repeat the tree
search.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Chris Mason <clm@fb.com>