staging: lustre: lnet/selftest: fix compile error on UP build

When compiled without CONFIG_SMP, we get a compile error
as ->ctb_parts is not defined.

There is already a function, cfs_cpt_cpumask(), which will get the
cpumask we need, and which handles the UP case by returning a NULL pointer.
So use that and handle NULL.
Also avoid the #ifdef by allocating a cpumask_var and copying
into it, rather than sharing the mask.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 6106c0f8 ("staging: lustre: lnet: convert selftest to use workqueues")
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>