netlink: validate NLA_MSECS length

commit c30bc94 upstream.

L2TP for example uses NLA_MSECS like this:
policy:
        [L2TP_ATTR_RECV_TIMEOUT]        = { .type = NLA_MSECS, },
code:
        if (info->attrs[L2TP_ATTR_RECV_TIMEOUT])
                cfg.reorder_timeout = nla_get_msecs(info->attrs[L2TP_ATTR_RECV_TIMEOUT]);

As nla_get_msecs() is essentially nla_get_u64() plus the
conversion to a HZ-based value, this will not properly
reject attributes from userspace that aren't long enough
and might overrun the message.

Add NLA_MSECS to the attribute minlen array to check the
size properly.

Cc: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>