[PATCH] create_workqueue_thread() signedness fix

With "-W -Wno-unused -Wno-sign-compare" I get the following compile warning:

  CC      kernel/workqueue.o
kernel/workqueue.c: In function `workqueue_cpu_callback':
kernel/workqueue.c:504: warning: ordered comparison of pointer with integer zero

On error create_workqueue_thread() returns NULL, not negative pointer, so
following trivial patch suggests itself.

Signed-off-by: Mika Kukkonen <mikukkon@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>