[IPV4]: Clean the ip_sockglue.c from some ugly ifdefs

The #idfed CONFIG_IP_MROUTE is sometimes places inside the if-s,
which looks completely bad. Similar ifdefs inside the functions
looks a bit better, but they are also not recommended to be used.

Provide an ifdef-ed ip_mroute_opt() helper to cleanup the code.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>