ipv6: Convert inet6_rtm_deladdr() to per-netns RTNL.

Let's register inet6_rtm_deladdr() with RTNL_FLAG_DOIT_PERNET and
hold rtnl_net_lock() before inet6_addr_del().

Now that inet6_addr_del() is always called under per-netns RTNL.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250115080608.28127-12-kuniyu@amazon.com


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