ipv6: Move nexthop_find_by_id() after fib6_info_alloc().

ipv6: Move nexthop_find_by_id() after fib6_info_alloc().

We will get rid of RTNL from RTM_NEWROUTE and SIOCADDRT.

Then, we must perform two lookups for nexthop and dev under RCU
to guarantee their lifetime.

ip6_route_info_create() calls nexthop_find_by_id() first if
RTA_NH_ID is specified, and then allocates struct fib6_info.

nexthop_find_by_id() must be called under RCU, but we do not want
to use GFP_ATOMIC for memory allocation here, which will be likely
to fail in ip6_route_multipath_add().

Let's move nexthop_find_by_id() after the memory allocation so
that we can later split ip6_route_info_create() into two parts:
the sleepable part and the RCU part.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/20250418000443.43734-6-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>