ip6mr: Add __init to ip6_mr_cleanup().

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: 3ac84e31b33e ("ipmr: Use rtnl_register_many().")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202410180139.B3HeemsC-lkp@intel.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20241017174732.39487-1-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>