Btrfs: remove BUG_ON() in btrfs_read_fs_tree_no_radix()

We have checked if ->node is NULL or not, so it is unnecessary to
use BUG_ON() to check again. Remove it.
Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>

fs/btrfs/disk-io.c

@@ -1513,7 +1513,6 @@ struct btrfs_root *btrfs_read_fs_root_no_radix(struct btrfs_root *tree_root,
 	}
 
 	root->commit_root = btrfs_root_node(root);
-	BUG_ON(!root->node); /* -ENOMEM */
 out:
 	if (location->objectid != BTRFS_TREE_LOG_OBJECTID) {
 		root->ref_cows = 1;