[IPV4]: Fix nexthop realm dumping for multipath routes

Routing realms exist per nexthop, but are only returned to userspace
for the first nexthop. This is due to the fact that iproute2 only
allows to set the realm for the first nexthop and the kernel refuses
multipath routes where only a single realm is present.

Dump all realms for multipath routes to enable iproute to correctly
display them.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>