Support setting IPV6_HOPLIMIT, IPV6_TCLASS, IPV6_DONTFRAG
during sendmsg via SOL_IPV6 cmsgs.

tclass and dontfrag are init'ed from struct ipv6_pinfo in
ipcm6_init_sk(), while hlimit is inited to -1, so we need
to handle it being populated via cmsg explicitly.

Leave extension headers and flowlabel unimplemented.
Those are slightly more laborious to test and users
seem to primarily care about IPV6_TCLASS.
Signed-off-by: NJakub Kicinski <kuba@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>