percpu-refcount: Don't use silly cmpxchg()

The cmpxchg() was just to ensure the debug check didn't race, which was
a bit excessive. The caller is supposed to do the appropriate
synchronization, which means percpu_ref_kill() can just do a simple
store.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>