btrfs: fix use of uninit "ret" in end_extent_writepage()

If this condition in end_extent_writepage() is false:

	if (tree->ops && tree->ops->writepage_end_io_hook)

we will then test an uninitialized "ret" at:

	ret = ret < 0 ? ret : -EIO;

The test for ret is for the case where ->writepage_end_io_hook failed, and we'd choose that ret as the error; but if there is no ->writepage_end_io_hook, nothing sets ret.

Initializing ret to 0 should be sufficient; if writepage_end_io_hook wasn't set, (!uptodate) means non-zero err was passed in, so we choose -EIO in that case.

---

p.s. - feel free to double check that this is sufficient ;)

index f25a909..20b73c4 100644
@@ -2354,7 +2354,7 @@ int end_extent_writepage(struct page *page, int err, u64 start, u64 end)
 {
 	int uptodate = (err == 0);
 	struct extent_io_tree *tree;
-	int ret;
+	int ret = 0;
 
 	tree = &BTRFS_I(page->mapping->host)->io_tree;