ip6mr: Add __init to ip6_mr_cleanup().

kernel test robot reported a section mismatch in ip6_mr_cleanup().

  WARNING: modpost: vmlinux: section mismatch in reference: ip6_mr_cleanup+0x0 (section: .text) -> 0xffffffff (section: .init.rodata)
  WARNING: modpost: vmlinux: section mismatch in reference: ip6_mr_cleanup+0x14 (section: .text) -> ip6mr_rtnl_msg_handlers (section: .init.rodata)

ip6_mr_cleanup() uses ip6mr_rtnl_msg_handlers[] that has
__initconst_or_module qualifier.

ip6_mr_cleanup() is only called from inet6_init() but does
not have __init qualifier.

Let's add __init to ip6_mr_cleanup().

Fixes: 3ac84e3 ("ipmr: Use rtnl_register_many().")
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Signed-off-by: Kuniyuki Iwashima <[email protected]>
Reviewed-by: Jacob Keller <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: q2ven

net/ipv6/ip6mr.c

@@ -1411,7 +1411,7 @@ int __init ip6_mr_init(void)
 	return err;
 }
 
-void ip6_mr_cleanup(void)
+void __init ip6_mr_cleanup(void)
 {
 	rtnl_unregister_many(ip6mr_rtnl_msg_handlers);
 #ifdef CONFIG_IPV6_PIMSM_V2