KVM: s390: fix calculation of idle_mask array size

We need BITS_TO_LONGS, not sizeof(long) to calculate
the correct size.

idle_mask is a bitmask, each bit representing the state
of a cpu. The desired outcome is an array of unsigned long
fields that can fit KVM_MAX_VCPUS bits. We should not use
sizeof(long) which returnes the size in bytes, but BITS_TO_LONGS

Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>