ipv6: mcast: Use in6_dev_get() in ipv6_dev_mc_dec().

ipv6: mcast: Use in6_dev_get() in ipv6_dev_mc_dec().

As well as __ipv6_dev_mc_inc(), all code in __ipv6_dev_mc_dec() are
protected by inet6_dev->mc_lock, and RTNL is not needed.

Let's use in6_dev_get() in ipv6_dev_mc_dec() and remove ASSERT_RTNL()
in __ipv6_dev_mc_dec().

Now, we can remove the RTNL comment above addrconf_leave_solict() too.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250702230210.3115355-6-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>