MPLS: Use mpls_features to activate software MPLS GSO segmentation

If an MPLS packet requires segmentation then use mpls_features
to determine if the software implementation should be used.

As no driver advertises MPLS GSO segmentation this will always be
the case.

I had not noticed that this was necessary before as software MPLS GSO
segmentation was already being used in my test environment. I believe that
the reason for that is the skbs in question always had fragments and the
driver I used does not advertise NETIF_F_FRAGLIST (which seems to be the
case for most drivers). Thus software segmentation was activated by
skb_gso_ok().

This introduces the overhead of an extra call to skb_network_protocol()
in the case where where CONFIG_NET_MPLS_GSO is set and
skb->ip_summed == CHECKSUM_NONE.

Thanks to Jesse Gross for prompting me to investigate this.

Signed-off-by: Simon Horman <horms@verge.net.au>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>