btrfs: fix uninitialized ret in ref-verify

Coverity caught a case where we could return with a uninitialized value
in ret in process_leaf.  This is actually pretty likely because we could
very easily run into a block group item key and have a garbage value in
ret and think there was an errror.  Fix this by initializing ret to 0.
Reported-by: Colin Ian King <colin.king@canonical.com>
Fixes: fd708b81 ("Btrfs: add a extent ref verify tool")
CC: stable@vger.kernel.org # 4.19+
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ref-verify.c

@@ -500,7 +500,7 @@ static int process_leaf(struct btrfs_root *root,
 	struct btrfs_extent_data_ref *dref;
 	struct btrfs_shared_data_ref *sref;
 	u32 count;
-	int i = 0, tree_block_level = 0, ret;
+	int i = 0, tree_block_level = 0, ret = 0;
 	struct btrfs_key key;
 	int nritems = btrfs_header_nritems(leaf);