mpls: Refactor how the mpls module is built

This refactoring is needed to allow more than just mpls gso
support to be built into the mpls moddule.

Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>