libceph: define mds_alloc_msg() method

The only user of the ceph messenger that doesn't define an alloc_msg
method is the mds client.  Define one, such that it works just like
it did before, and simplify ceph_con_in_msg_alloc() by assuming the
alloc_msg method is always present.

This and the next patch resolve:
    http://tracker.ceph.com/issues/4322

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 4efbc63..b87b24f 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -3473,6 +3473,28 @@
 	return ceph_monc_validate_auth(&mdsc->fsc->client->monc);
 }
 
+static struct ceph_msg *mds_alloc_msg(struct ceph_connection *con,
+				struct ceph_msg_header *hdr, int *skip)
+{
+	struct ceph_msg *msg;
+	int type = (int) le16_to_cpu(hdr->type);
+	int front_len = (int) le32_to_cpu(hdr->front_len);
+
+	if (con->in_msg)
+		return con->in_msg;
+
+	*skip = 0;
+	msg = ceph_msg_new(type, front_len, GFP_NOFS, false);
+	if (!msg) {
+		pr_err("unable to allocate msg type %d len %d\n",
+		       type, front_len);
+		return NULL;
+	}
+	msg->page_alignment = (unsigned int) le16_to_cpu(hdr->data_off);
+
+	return msg;
+}
+
 static const struct ceph_connection_operations mds_con_ops = {
 	.get = con_get,
 	.put = con_put,
@@ -3481,6 +3503,7 @@
 	.verify_authorizer_reply = verify_authorizer_reply,
 	.invalidate_authorizer = invalidate_authorizer,
 	.peer_reset = peer_reset,
+	.alloc_msg = mds_alloc_msg,
 };
 
 /* eof */