libceph: clean up skipped message logic

In ceph_con_in_msg_alloc() it is possible for a connection's
alloc_msg method to indicate an incoming message should be skipped.
By default, read_partial_message() initializes the skip variable
to 0 before it gets provided to ceph_con_in_msg_alloc().

The osd client, mon client, and mds client each supply an alloc_msg
method.  The mds client always assigns skip to be 0.

The other two leave the skip value of as-is, or assigns it to zero,
except:
    - if no (osd or mon) request having the given tid is found, in
      which case skip is set to 1 and NULL is returned; or
    - in the osd client, if the data of the reply message is not
      adequate to hold the message to be read, it assigns skip
      value 1 and returns NULL.
So the returned message pointer will always be NULL if skip is ever
non-zero.

Clean up the logic a bit in ceph_con_in_msg_alloc() to make this
state of affairs more obvious.  Add a comment explaining how a null
message pointer can mean either a message that should be skipped or
a problem allocating a message.

This resolves:
    http://tracker.ceph.com/issues/4324

Reported-by: Greg Farnum <greg@inktank.com>
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>

net/ceph/messenger.c

@@ -2819,18 +2819,21 @@ static int ceph_con_in_msg_alloc(struct ceph_connection *con, int *skip)
 			ceph_msg_put(msg);
 		return -EAGAIN;
 	}
-	con->in_msg = msg;
-	if (con->in_msg) {
+	if (msg) {
+		BUG_ON(*skip);
+		con->in_msg = msg;
 		con->in_msg->con = con->ops->get(con);
 		BUG_ON(con->in_msg->con == NULL);
-	}
-	if (*skip) {
-		con->in_msg = NULL;
-		return 0;
-	}
-	if (!con->in_msg) {
-		con->error_msg =
-			"error allocating memory for incoming message";
+	} else {
+		/*
+		 * Null message pointer means either we should skip
+		 * this message or we couldn't allocate memory.  The
+		 * former is not an error.
+		 */
+		if (*skip)
+			return 0;
+		con->error_msg = "error allocating memory for incoming message";
+
 		return -ENOMEM;
 	}
 	memcpy(&con->in_msg->hdr, &con->in_hdr, sizeof(con->in_hdr));