Use layer five checksum flags in the mbuf packet header to pass on crypto state.

The mbuf protocol flags get cleared between layers, and also it was discovered
that M_DECRYPTED conflicts with M_HASFCS when receiving ethernet patckets.

Add the proper CSUM_TLS_MASK and CSUM_TLS_DECRYPTED defines, and start using
these instead of M_DECRYPTED inside the TCP LRO code.

This change is needed by coming TLS RX hardware offload support patches.

Suggested by: kib@
Reviewed by: jhb@
MFC after: 1 week
Sponsored by: NVIDIA Networking