LU-9859 libcfs: switch to cpumask_var_t

LU-9859 libcfs: 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.

Linux-commit: 3872fb73cabdd47fd4abf7b6eff21d06e57297eb

Change-Id: I58d3f2e1fb1c71e1bd094a60ec4eb49b477e69e3
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-on: https://review.whamcloud.com/35809
Reviewed-by: Neil Brown <neilb@suse.de>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>