f2fs: set cached_en after checking finally

This patch relocates cached_en not only to be covered by spin_lock, but also
to set once after checking out completely.

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Samuel Pascua <pascua.samuel.14@gmail.com>