netlabel: Prevent setsockopt() from changing the hop-by-hop option.

If a socket has a netlabel in place then don't let setsockopt() alter
the socket's IPv6 hop-by-hop option.  This is in the same spirit as
the existing check for IPv4.

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