rip6: Fix handling of checksums in rip6_input()

[markj]

Summary

A v6 raw socket may ask the kernel to validate the checksum of an
inbound packet. If so, and the validation fails, we discard the packet,
but this isn't really right: other raw sockets may wish to receive a
copy of the packet anyway.

Rework checksum handling to address this problem, and use a flag to
avoid computing the checksum more than once for a given packet.

Fixes: de2d47842e880281 ("SMR protection for inpcbs")

[pouria]

LGTM

[glebius]

I'd suggest to change commit header to "raw ip6: Fix handling of checksums in rip6_input()", or "rawip: ...". The "rip" abbreviation can be easily confused with routing protocol. Unfortunately, I already made a precedent in the past with "rip: " header. And you already pushed changes with a better "rawip: " header to this file. The IPv4 sibling has quite a long precedence of using "raw ip: ".