Btrfs: fix uninit variable in repair_eb_io_failure

We'd have to be passing bogus extent buffers for this uninit variable to
actually be used, but set it to zero just in case.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_io.c

@@ -1937,7 +1937,7 @@ int repair_eb_io_failure(struct btrfs_root *root, struct extent_buffer *eb,
 	struct btrfs_mapping_tree *map_tree = &root->fs_info->mapping_tree;
 	u64 start = eb->start;
 	unsigned long i, num_pages = num_extent_pages(eb->start, eb->len);
-	int ret;
+	int ret = 0;
 
 	for (i = 0; i < num_pages; i++) {
 		struct page *p = extent_buffer_page(eb, i);