mppc - Finish pluging NETGRAPH_MPPC_COMPRESSION.

There were several places where reference to compression were left
unfinished. Furthermore, KASSERTs contained references to MPPC_INVALID
which is not defined in the tree and therefore were sure to break with
INVARIANTS: comment them out.

Reported by: Eugene Grosbein
PR: 216265
MFC after: 3 days