f2fs: remove unnecessary condition judgment

Remove the unnecessary condition judgment, because
'max_slots' has been initialized to '0' at the beginging
of the function, as following:
if (max_slots)
       *max_slots = 0;

Signed-off-by: Yuan Zhong <yuan.mark.zhong@samsung.com>
Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>