f2fs: avoid free slab cache under spinlock

Move kmem_cache_free out of spinlock protection region for better performance.

Change log from v1:
 o remove spinlock protection for kmem_cache_free in destroy_node_manager
suggested by Jaegeuk Kim.

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