s390/ctcm: fix potential memory leak

smatch complains about
drivers/s390/net/ctcm_mpc.c:1210 ctcmpc_unpack_skb() warn: possible memory leak of 'mpcginfo'

mpc_action_discontact() did not free mpcginfo. Consolidate the freeing in
ctcmpc_unpack_skb().

Fixes: 293d984f

 ("ctcm: infrastructure for replaced ctc driver")
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>