staging: erofs: fix `erofs_workgroup_{try_to_freeze, unfreeze}'

There are two minor issues in the current freeze interface:

   1) Freeze interfaces have not related with CONFIG_DEBUG_SPINLOCK,
      therefore fix the incorrect conditions;

   2) For SMP platforms, it should also disable preemption before
      doing atomic_cmpxchg in case that some high priority tasks
      preempt between atomic_cmpxchg and disable_preempt, then spin
      on the locked refcount later.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>