f2fs: return the tmp_ptr directly in __bitmap_ptr

Just return tmp_ptr here, it's no need to dereference
checkpoint pointer again.

Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>