util: check accept_ra for all nexthop interfaces of multipath routes

When checking the setting of accept_ra, we have assumed that all
routes have a single nexthop, so the interface of the route would be
in the RTA_OIF attribute of the netlink RTM_NEWROUTE message. But
multipath routes don't have an RTA_OIF; instead, they have an
RTA_MULTIPATH attribute, which is an array of rtnexthop, with each
rtnexthop having an interface. This patch adds a loop to look at the
setting of accept_ra of the interface for every rtnexthop in the
array.

Signed-off-by: Laine Stump <laine@laine.org>
Reviewed-by: Erik Skultety <eskultet@redhat.com>