ipv6: Check GATEWAY in rtm_to_fib6_multipath_config().

ipv6: Check GATEWAY in rtm_to_fib6_multipath_config().

In ip6_route_multipath_add(), we call rt6_qualify_for_ecmp() for each
entry.  If it returns false, the request fails.

rt6_qualify_for_ecmp() returns false if either of the conditions below
is true:

  1. f6i->fib6_flags has RTF_ADDRCONF
  2. f6i->nh is not NULL
  3. f6i->fib6_nh->fib_nh_gw_family is AF_UNSPEC

1 is unnecessary because rtm_to_fib6_config() never sets RTF_ADDRCONF
to cfg->fc_flags.

2. is equivalent with cfg->fc_nh_id.

3. can be replaced by checking RTF_GATEWAY in the base and each multipath
entry because AF_INET6 is set to f6i->fib6_nh->fib_nh_gw_family only when
cfg.fc_is_fdb is true or RTF_GATEWAY is set, but the former is always
false.

These checks do not require RCU and can be done earlier.

Let's perform the equivalent checks in rtm_to_fib6_multipath_config().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250418000443.43734-5-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>