f2fs: shrink size of struct discard_cmd

In order to shrink size of struct discard_cmd, change variable type of
@state in struct discard_cmd from int to unsigned char.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -220,7 +220,7 @@ struct discard_cmd {
 	struct completion wait;		/* compleation */
 	struct block_device *bdev;	/* bdev */
 	unsigned short ref;		/* reference count */
-	int state;			/* state */
+	unsigned char state;		/* state */
 	int error;			/* bio error */
 };