erofs: fix return value check in erofs_read_superblock()

In case of error, the function read_mapping_page() returns
ERR_PTR() not NULL. The NULL test in the return value check
should be replaced with IS_ERR().

Fixes: fe7c2423 ("erofs: use read_mapping_page instead of sb_bread")
Reviewed-by: NGao Xiang <gaoxiang25@huawei.com>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20190918083033.47780-1-weiyongjun1@huawei.comSigned-off-by: NGao Xiang <gaoxiang25@huawei.com>

fs/erofs/super.c

@@ -105,9 +105,9 @@ static int erofs_read_superblock(struct super_block *sb)
 	int ret;
 
 	page = read_mapping_page(sb->s_bdev->bd_inode->i_mapping, 0, NULL);
-	if (!page) {
+	if (IS_ERR(page)) {
 		erofs_err(sb, "cannot read erofs superblock");
-		return -EIO;
+		return PTR_ERR(page);
 	}
 
 	sbi = EROFS_SB(sb);