f2fs: fix to return SHRINK_EMPTY if no objects to free

Quoted from include/linux/shrinker.h
"count_objects should return the number of freeable items in the cache. If
 there are no objects to free, it should return SHRINK_EMPTY, while 0 is
 returned in cases of the number of freeable items cannot be determined
 or shrinker should skip this cache for this time (e.g., their number
 is below shrinkable limit)."

Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>