nexthop: Move NHA_OIF validation to rtm_to_nh_config_rtnl().

NHA_OIF needs to look up a device by __dev_get_by_index(),
which requires RTNL.

Let's move NHA_OIF validation to rtm_to_nh_config_rtnl().

Note that the proceeding checks made the original !cfg->nh_fdb
check redundant.

  NHA_FDB is set           -> NHA_OIF cannot be set
  NHA_FDB is set but false -> NHA_OIF must be set
  NHA_FDB is not set       -> NHA_OIF must be set

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250319230743.65267-4-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>