net: ping6: support setting socket options via cmsg

Minor reordering of the code and a call to sock_cmsg_send()
gives us support for setting the common socket options via
cmsg (the usual ones - SO_MARK, SO_TIMESTAMPING_OLD, SCM_TXTIME).

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>