btrfs-progs: take a ref in the root locking code

This code in the kernel not only returns the locked root, but also takes
a reference on the node.  This is important for when we sync ctree.c
into btrfs-progs, it expects that references are held on the root node
after calling these helpers.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: josefbacik

kernel-shared/locking.c

@@ -5,15 +5,18 @@
 
 struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
 {
+	root->node->refs++;
 	return root->node;
 }
 
 struct extent_buffer *btrfs_try_read_lock_root_node(struct btrfs_root *root)
 {
+	root->node->refs++;
 	return root->node;
 }
 
 struct extent_buffer *btrfs_lock_root_node(struct btrfs_root *root)
 {
+	root->node->refs++;
 	return root->node;
 }