staging/lustre/libcfs: replace deprecated cpus_ calls with cpumask_

Rusty Russel advises that cpus_* functions are deprecated to work
on cpumasks and cpumask_* functions should be called instead,
otherwise problems with CPUMASK_OFFSTACK arise.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>