ipv6: fix EFAULT on sendto with icmpv6 and hdrincl

[Nicolas Dichtel]

Hi Ubuntu kernel team,

please, can you consider to cherry-pick the following linux upstream fixes:
59e3e4b52663 ("ipv6: use READ_ONCE() for inet->hdrincl as in ipv4")
b9aa52c4cb45 ("ipv6: fix EFAULT on sendto with icmpv6 and hdrincl")

They have been included in v5.2 and the patch that introduces the bug has been
included in v4.5.

Here is the launchpad entry:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1832828


Thank you,
Nicolas