workqueue: Use IS_ERR and PTR_ERR instead of PTR_ERR_OR_ZERO.

Replace inline function PTR_ERR_OR_ZERO with IS_ERR and PTR_ERR to
remove redundant parameter definitions and checks.
Reduce code size.
Before:
   text	   data	    bss	    dec	    hex	filename
  47510	   5979	    840	  54329	   d439	kernel/workqueue.o
After:
   text	   data	    bss	    dec	    hex	filename
  47474	   5979	    840	  54293	   d415	kernel/workqueue.o

Signed-off-by: Sean Fu <fxinrong@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>