route: filter by interface on multipath routes

route: filter by interface on multipath routes

The ip route command would silently hide multipath routes when filter
by interface. The problem was it was not looking for interface when
filter multipath routes.

Example:
	ip link add name dummy1 up type dummy
	ip link add name dummy2 up type dummy
	ip address add 192.0.2.1/28 dev dummy1
	ip address add 192.0.2.17/28 dev dummy2
	ip route add 198.51.100.0/24 \
		nexthop via 192.0.2.2 dev dummy1 \
		nexthop via 192.0.2.18 dev dummy2

Before:
ip route show dev dummy1
192.0.2.0/28 proto kernel scope link src 192.0.2.1

After:
ip route show dev dummy1
192.0.2.0/28 proto kernel scope link src 192.0.2.1
198.51.100.0/24
	nexthop via 192.0.2.2 dev dummy1 weight 1
	nexthop via 192.0.2.18 dev dummy2 weight 1

Reported-by: "Muggeridge, Matt" <matt.muggeridge2@hpe.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>