libceph: make message data be a pointer

Begin the transition from a single message data item to a list of
them by replacing the "data" structure in a message with a pointer
to a ceph_msg_data structure.

A null pointer will indicate the message has no data; replace the
use of ceph_msg_has_data() with a simple check for a null pointer.

Create functions ceph_msg_data_create() and ceph_msg_data_destroy()
to dynamically allocate and free a data item structure of a given type.

When a message has its data item "set," allocate one of these to
hold the data description, and free it when the last reference to
the message is dropped.

This partially resolves:
    http://tracker.ceph.com/issues/4429

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

include/linux/ceph/messenger.h

@@ -64,8 +64,6 @@ struct ceph_messenger {
 	u32 required_features;
 };
 
-#define ceph_msg_has_data(m)		((m)->data.type != CEPH_MSG_DATA_NONE)
-
 enum ceph_msg_data_type {
 	CEPH_MSG_DATA_NONE,	/* message contains no data payload */
 	CEPH_MSG_DATA_PAGES,	/* data source/destination is a page array */
@@ -141,8 +139,7 @@ struct ceph_msg {
 	struct kvec front;              /* unaligned blobs of message */
 	struct ceph_buffer *middle;
 
-	/* data payload */
-	struct ceph_msg_data	data;
+	struct ceph_msg_data	*data;	/* data payload */
 
 	struct ceph_connection *con;
 	struct list_head list_head;	/* links for connection lists */