[XFRM]: Fix XFRMGRP_REPORT to use correct multicast group.

XFRMGRP_REPORT uses 0x10 which is a group that belongs
to events. The correct value is 0x20.
We should really be using xfrm_nlgroups going forward; it was tempting
to delete the definition of XFRMGRP_REPORT but it would break at
least iproute2.

Signed-off-by: J Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>