ipv6: sr: add support for SRH injection through setsockopt

This patch adds support for per-socket SRH injection with the setsockopt
system call through the IPPROTO_IPV6, IPV6_RTHDR options.
The SRH is pushed through the ipv6_push_nfrag_opts function.

Signed-off-by: David Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>