ipv6: Split ip6_route_info_create().

ipv6: Split ip6_route_info_create().

We will get rid of RTNL from RTM_NEWROUTE and SIOCADDRT and rely
on RCU to guarantee dev and nexthop lifetime.

Then, we want to allocate as much as possible before entering
the RCU section.

The RCU section will start in the middle of ip6_route_info_create(),
and this is problematic for ip6_route_multipath_add() that calls
ip6_route_info_create() multiple times.

Let's split ip6_route_info_create() into two parts; one for memory
allocation and another for nexthop setup.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/20250418000443.43734-7-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>