ipv6: Convert inet6_rtm_newaddr() to per-netns RTNL.

Let's register inet6_rtm_newaddr() with RTNL_FLAG_DOIT_PERNET
and hold rtnl_net_lock() before __dev_get_by_index().

Now that inet6_addr_add() and inet6_addr_modify() are always
called under per-netns RTNL.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250115080608.28127-11-kuniyu@amazon.com


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