f2fs: switch to check FI_NO_EXTENT in f2fs_{lookup,update}_extent_cache

This patch switch to check FI_NO_EXTENT in f2fs_{lookup,update}_extent_cache
instead of f2fs_{lookup,update}_extent_tree or {lookup,update}_extent_info.

No functionality modification in this patch.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>