mbuf checksum flags and fields to support tunneling protocols.

These are being added to support VXLAN but will work for GENEVE as well.
ENCAP_RSVD1 will likely become ENCAP_GENEVE in the future.

The size of struct mbuf does not change and that means this change can be MFC'd.
If size wasn't a constraint a cleaner way may have been to add inner_csum_flags
and inner_csum_data to go with csum_flags and csum_data.

Reviewed by: kib@
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D25873