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/net/ceph/messenger.c b/net/ceph/messenger.c
index 770c527..bad3d4a 100644
--- a/net/ceph/messenger.c
+++ b/net/ceph/messenger.c
@@ -3425,7 +3425,7 @@ static void ceph_msg_release(struct kref *kref)
 struct ceph_msg *ceph_msg_get(struct ceph_msg *msg)
 {
 	dout("%s %p (was %d)\n", __func__, msg,
-	     atomic_read(&msg->kref.refcount));
+	     kref_read(&msg->kref));
 	kref_get(&msg->kref);
 	return msg;
 }
@@ -3434,7 +3434,7 @@ EXPORT_SYMBOL(ceph_msg_get);
 void ceph_msg_put(struct ceph_msg *msg)
 {
 	dout("%s %p (was %d)\n", __func__, msg,
-	     atomic_read(&msg->kref.refcount));
+	     kref_read(&msg->kref));
 	kref_put(&msg->kref, ceph_msg_release);
 }
 EXPORT_SYMBOL(ceph_msg_put);