ipv6/multipath: remove flag NLM_F_EXCL after the first nexthop

fib6_add_rt2node() will reject the nexthop if this flag is set, so
we perform the check only for the first nexthop.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>