staging: lustre: lnet: switch to cpumask_var_t

So that we can use the common cpumask allocation functions,
switch to cpumask_var_t.
We need to be careful not to free a cpumask_var_t until the
variable has been initialized, and it cannot be initialized
directly.
So we must be sure either that it is filled with zeros, or
that zalloc_cpumask_var() has been called on it.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>