btrfs: change BUG_ON to assertion when checking for delayed_node root

[ Upstream commit be73f4448b607e6b7ce41cd8ef2214fdf6e7986f ]

The pointer to root is initialized in btrfs_init_delayed_node(), no need
to check for it again. Change the BUG_ON to assertion.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>