btrfs: use free_root_extent_buffer to free root

In btrfs_put_root() we're freeing a btrfs_root's 'node' and 'commit_root'
extent buffers manually via kfree(), while we're using
free_root_extent_buffers() in the free_root_pointers() function above.

free_root_extent_buffers() also NULLs the pointers after freeing, which
mitigates potential double frees.
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -2001,8 +2001,7 @@ void btrfs_put_root(struct btrfs_root *root)
 		if (root->anon_dev)
 			free_anon_bdev(root->anon_dev);
 		btrfs_drew_lock_destroy(&root->snapshot_lock);
-		free_extent_buffer(root->node);
-		free_extent_buffer(root->commit_root);
+		free_root_extent_buffers(root);
 		kfree(root->free_ino_ctl);
 		kfree(root->free_ino_pinned);
 #ifdef CONFIG_BTRFS_DEBUG