[IPV6]: Fix memory leak in cleanup_ipv6_mibs()

The icmpv6msg mib statistics is not freed.

This is almost not critical for current kernel, since ipv6
module is unloadable, but this can happen on load error and 
will happen every time we stop the network namespace (when 
we have one, of course).

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>