locking/atomic, kref: Add kref_read()

Since we need to change the implementation, stop exposing internals.

Provide kref_read() to read the current reference count; typically
used for debug messages.

Kills two anti-patterns:

	atomic_read(&kref->refcount)
	kref->refcount.counter

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/include/linux/sunrpc/cache.h b/include/linux/sunrpc/cache.h
index 62a60ee..8a511c0 100644
--- a/include/linux/sunrpc/cache.h
+++ b/include/linux/sunrpc/cache.h
@@ -198,7 +198,7 @@ static inline struct cache_head  *cache_get(struct cache_head *h)
 
 static inline void cache_put(struct cache_head *h, struct cache_detail *cd)
 {
-	if (atomic_read(&h->ref.refcount) <= 2 &&
+	if (kref_read(&h->ref) <= 2 &&
 	    h->expiry_time < cd->nextcheck)
 		cd->nextcheck = h->expiry_time;
 	kref_put(&h->ref, cd->cache_put);