ipv6: Add ipv6_renew_options_kern() that accepts a kernel mem pointer.

The functionality is equivalent to ipv6_renew_options() except
that the newopt pointer is in kernel, not user, memory

The kernel memory implementation will be used by the CALIPSO network
labelling engine, which needs to be able to set IPv6 hop-by-hop
options.

Signed-off-by: Huw Davies <huw@codeweavers.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>