ipv6: Move some validation from ip6_route_info_create() to rtm_to_fib6_config().

ipv6: Move some validation from ip6_route_info_create() to rtm_to_fib6_config().

ip6_route_info_create() is called from 3 functions:

  * ip6_route_add()
  * ip6_route_multipath_add()
  * addrconf_f6i_alloc()

addrconf_f6i_alloc() does not need validation for struct fib6_config in
ip6_route_info_create().

ip6_route_multipath_add() calls ip6_route_info_create() for multiple
routes with slightly different fib6_config instances, which is copied
from the base config passed from userspace.  So, we need not validate
the same config repeatedly.

Let's move such validation into rtm_to_fib6_config().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/20250418000443.43734-4-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>