ip/ipmroute: use preferred_family to get prefix

ip/ipmroute: use preferred_family to get prefix

The mroute family is reset to RTNL_FAMILY_IPMR or RTNL_FAMILY_IP6MR when
retrieving the multicast routing cache. However, the get_prefix() and
subsequently __get_addr_1() cannot identify these families. Using
preferred_family to obtain the prefix can resolve this issue.

Fixes: 98ce99273f24 ("mroute: fix up family handling")
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>