percpu-refcount: implement percpu_ref_cancel_init()

Normally, percpu_ref_init() initializes and percpu_ref_kill()
initiates destruction which completes asynchronously.  The
asynchronous destruction can be problematic in init failure path where
the caller wants to destroy half-constructed object - distinguishing
half-constructed objects from the usual release method can be painful
for complex objects.

This patch implements percpu_ref_cancel_init() which synchronously
destroys the percpu_ref without invoking release.  To avoid
unintentional misuses, the function requires the ref to have finished
percpu_ref_init() but never used and triggers WARN otherwise.

v2: Explain the weird name and usage restriction in the function
    comment.

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

include/linux/percpu-refcount.h

@@ -68,6 +68,7 @@ struct percpu_ref {
 
 int __must_check percpu_ref_init(struct percpu_ref *ref,
 				 percpu_ref_func_t *release);
+void percpu_ref_cancel_init(struct percpu_ref *ref);
 void percpu_ref_kill(struct percpu_ref *ref);
 
 #define PCPU_STATUS_BITS	2