s390/diag: add diag26c support for VNIC info

With subcode 0x24, diag26c returns all sorts of VNIC-related information.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>