workqueue: use zalloc_cpumask_var() for gcwq->mayday_mask

alloc_mayday_mask() was using alloc_cpumask_var() making
gcwq->mayday_mask contain garbage after initialization on
CONFIG_CPUMASK_OFFSTACK=y configurations.  This combined with the
previously fixed GCWQ_DISASSOCIATED initialization bug could make
rescuers fall into infinite loop trying to bind to an offline cpu.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: CAI Qian <caiqian@redhat.com>