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: Chris Mason <[e-mail]>

fs/btrfs/ordered-data.c

···
 					  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;