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 <chris.mason@oracle.com>
diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
index 2b61e1d..083a554 100644
--- a/fs/btrfs/ordered-data.c
+++ b/fs/btrfs/ordered-data.c
@@ -141,7 +141,7 @@
 					  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;