net: ipv4: fix sparse error in fib_good_nh()

Fixes following sparse errors :

net/ipv4/fib_semantics.c:1579:61: warning: incorrect type in argument 2
(different base types)
net/ipv4/fib_semantics.c:1579:61:    expected unsigned int [unsigned]
[usertype] key
net/ipv4/fib_semantics.c:1579:61:    got restricted __be32 const
[usertype] nh_gw

Fixes: a6db4494

 ("net: ipv4: Consider failed nexthops in multipath routes")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>