ipv6: Validate RTA_GATEWAY of RTA_MULTIPATH in rtm_to_fib6_config().

ipv6: Validate RTA_GATEWAY of RTA_MULTIPATH in rtm_to_fib6_config().

We will perform RTM_NEWROUTE and RTM_DELROUTE under RCU, and then
we want to perform some validation out of the RCU scope.

When creating / removing an IPv6 route with RTA_MULTIPATH,
inet6_rtm_newroute() / inet6_rtm_delroute() validates RTA_GATEWAY
in each multipath entry.

Let's do that in rtm_to_fib6_config().

Note that now RTM_DELROUTE returns an error for RTA_MULTIPATH with
0 entries, which was accepted but should result in -EINVAL as
RTM_NEWROUTE.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250418000443.43734-2-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>