MFC r272404:
Fix the checksum computation for UDPLite/IPv6. This requires the
usage of a function computing the checksum only over a part of the function.
Therefore introduce in6_cksum_partial() and implement in6_cksum() based
on that.
While there, ensure that the UDPLite packet contains at least enough bytes
to contain the header.

Merge r263091: fix mbuf flags clash that lead to failure of operation
of IPSEC and packet filters.

PR: kern/185876
PR: kern/186755

MFC r272404:
Fix the checksum computation for UDPLite/IPv6. This requires the
usage of a function computing the checksum only over a part of the function.
Therefore introduce in6_cksum_partial() and implement in6_cksum() based
on that.
While there, ensure that the UDPLite packet contains at least enough bytes
to contain the header.

Merge r263091: fix mbuf flags clash that lead to failure of operation
of IPSEC and packet filters.

PR: kern/185876
PR: kern/186755