include/linux/mm.h: return true in cpupid_pid_unset()

Fix the following coccicheck warning:

  include/linux/mm.h:1371:8-9: WARNING: return of 0/1 in function 'cpupid_pid_unset' with return type bool

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200422071816.48879-1-yanaijie@huawei.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>