Because of the extent shrinker or other -ENOMEM scenarios, it cannot guarantee
that the largest extent would be cached in the tree all the time.

Instead of relying on extent_tree, we can simply check the cached one in extent
tree accordingly.
Reviewed-by: NChao Yu <chao2.yu@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>