sch_htb: Add extack messages for EOPNOTSUPP errors

In order to make the "Operation not supported" message clearer to the
user, add extack messages explaining why exactly adding offloaded HTB
could be not supported in each case.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>