netinet6: Implement in6_cksum_partial() using m_apply()

[markj]

Summary

This ensures that in6_cksum_partial() can be applied to unmapped mbufs,
which can happen at least when icmp6_reflect() quotes a packet.

The basic idea is to restructure in6_cksum_partial() to operate on one
mbuf at a time. If the buffer length is odd or unaligned, an extra
residual byte may be returned, to be incorporated into the checksum when
processing the next buffer.

PR: 268400

Test Plan

I ported both versions to run in userspace, then wrote a little harness
to generate a sequence of random packets and compute checksums
generated by the two implementations. They all matched on 100000
iterations of the test.

[cy]

I see no problems with ipfilter.