ibmveth: consolidate kmalloc of array, memset 0 to kcalloc

This is an API consolidation only. The use of kmalloc + memset to 0
is equivalent to kcalloc in this case as it is allocating an array
of elements.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>