ipc: modify message queue accounting to not take kernel data structures into account

A while back, the message queue implementation in the kernel was
improved to use btrees to speed up retrieval of messages, in commit
d6629859 ("ipc/mqueue: improve performance of send/recv").

That patch introducing the improved kernel handling of message queues
(using btrees) has, as a by-product, changed the meaning of the QSIZE
field in the pseudo-file created for the queue.  Before, this field
reflected the size of the user-data in the queue.  Since, it also takes
kernel data structures into account.  For example, if 13 bytes of user
data are in the queue, on my machine the file reports a size of 61
bytes.

There was some discussion on this topic before (for example
https://lkml.org/lkml/2014/10/1/115).  Commenting on a th lkml, Michael
Kerrisk gave the following background
(https://lkml.org/lkml/2015/6/16/74):

    The pseudofiles in the mqueue filesystem (usually mounted at
    /dev/mqueue) expose fields with me...

ipc/mqueue.c

@@ -142,7 +142,6 @@ static int msg_insert(struct msg_msg *msg, struct mqueue_inode_info *info)
 		if (!leaf)
 			return -ENOMEM;
 		INIT_LIST_HEAD(&leaf->msg_list);
-		info->qsize += sizeof(*leaf);
 	}
 	leaf->priority = msg->m_type;
 	rb_link_node(&leaf->rb_node, parent, p);
@@ -187,7 +186,6 @@ static inline struct msg_msg *msg_get(struct mqueue_inode_info *info)
 			     "lazy leaf delete!\n");
 		rb_erase(&leaf->rb_node, &info->msg_tree);
 		if (info->node_cache) {
-			info->qsize -= sizeof(*leaf);
 			kfree(leaf);
 		} else {
 			info->node_cache = leaf;
@@ -200,7 +198,6 @@ static inline struct msg_msg *msg_get(struct mqueue_inode_info *info)
 		if (list_empty(&leaf->msg_list)) {
 			rb_erase(&leaf->rb_node, &info->msg_tree);
 			if (info->node_cache) {
-				info->qsize -= sizeof(*leaf);
 				kfree(leaf);
 			} else {
 				info->node_cache = leaf;
@@ -1034,7 +1031,6 @@ SYSCALL_DEFINE5(mq_timedsend, mqd_t, mqdes, const char __user *, u_msg_ptr,
 		/* Save our speculative allocation into the cache */
 		INIT_LIST_HEAD(&new_leaf->msg_list);
 		info->node_cache = new_leaf;
-		info->qsize += sizeof(*new_leaf);
 		new_leaf = NULL;
 	} else {
 		kfree(new_leaf);
@@ -1142,7 +1138,6 @@ SYSCALL_DEFINE5(mq_timedreceive, mqd_t, mqdes, char __user *, u_msg_ptr,
 		/* Save our speculative allocation into the cache */
 		INIT_LIST_HEAD(&new_leaf->msg_list);
 		info->node_cache = new_leaf;
-		info->qsize += sizeof(*new_leaf);
 	} else {
 		kfree(new_leaf);
 	}