f2fs: set raw_super default to NULL to avoid compile warning

Set raw_super default to NULL to avoid the possibly used
uninitialized warning, though we may never hit it in fact.
Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -930,7 +930,7 @@ static int read_raw_super_block(struct super_block *sb,
 static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 {
 	struct f2fs_sb_info *sbi;
-	struct f2fs_super_block *raw_super;
+	struct f2fs_super_block *raw_super = NULL;
 	struct buffer_head *raw_super_buf;
 	struct inode *root;
 	long err = -EINVAL;