libceph: wrap auth methods in a mutex

The auth code is called from a variety of contexts, include the mon_client
(protected by the monc's mutex) and the messenger callbacks (currently
protected by nothing).  Avoid chaos by protecting all auth state with a
mutex.  Nothing is blocking, so this should be simple and lightweight.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>

include/linux/ceph/auth.h

@@ -78,6 +78,8 @@ struct ceph_auth_client {
 	u64 global_id;          /* our unique id in system */
 	const struct ceph_crypto_key *key;     /* our secret key */
 	unsigned want_keys;     /* which services we want */
+
+	struct mutex mutex;
 };
 
 extern struct ceph_auth_client *ceph_auth_init(const char *name,