f2fs: correct type of wait in struct bio_private

The void *wait in bio_private is used for waiting completion of checkpoint bio.
So we don't need to use its type as void, but declare it as completion type.
Signed-off-by: NChao Yu <chao2.yu@samsung.com>
[Jaegeuk Kim: add description]
Signed-off-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/segment.h

@@ -96,7 +96,7 @@
 struct bio_private {
 	struct f2fs_sb_info *sbi;
 	bool is_sync;
-	void *wait;
+	struct completion *wait;
 };
 
 /*