bridge: netlink: add support for igmp's intervals

Add support to set/get all of the igmp's configurable intervals via
netlink. These currently are:
IFLA_BR_MCAST_LAST_MEMBER_INTVL
IFLA_BR_MCAST_MEMBERSHIP_INTVL
IFLA_BR_MCAST_QUERIER_INTVL
IFLA_BR_MCAST_QUERY_INTVL
IFLA_BR_MCAST_QUERY_RESPONSE_INTVL
IFLA_BR_MCAST_STARTUP_QUERY_INTVL

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>