ieee802154: 6lowpan: Validate link netns in newlink() of rtnl_link_ops

ieee802154: 6lowpan: Validate link netns in newlink() of rtnl_link_ops

Device denoted by IFLA_LINK is in link_net (IFLA_LINK_NETNSID) or
source netns by design, but 6lowpan uses dev_net.

Note dev->netns_local is set to true and currently link_net is
implemented via a netns change. These together effectively reject
IFLA_LINK_NETNSID.

This patch adds a validation to ensure link_net is either NULL or
identical to dev_net. Thus it would be fine to continue using dev_net
when rtnetlink core begins to create devices directly in target netns.

Signed-off-by: Xiao Liang <shaw.leon@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250219125039.18024-5-shaw.leon@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>