Btrfs: avoid uninit variable warnings in ordered-data.c

This one isn't really an uninit variable, but for pretty
obscure reasons.  Let's make it clearly correct.
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/ordered-data.c

@@ -141,7 +141,7 @@ static inline struct rb_node *tree_search(struct btrfs_ordered_inode_tree *tree,
 					  u64 file_offset)
 {
 	struct rb_root *root = &tree->tree;
-	struct rb_node *prev;
+	struct rb_node *prev = NULL;
 	struct rb_node *ret;
 	struct btrfs_ordered_extent *entry;