staging: lustre: replace kmalloc with kmalloc_array

This patch fixes the following checkpatch.pl warning:

WARNING: Prefer kmalloc_array over kmalloc with multiply

Signed-off-by: Denis Petrovic <denis.petrovic@edu.ece.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>