xen/xenbus: fix return type in xenbus_file_read()

This code tries to store -EFAULT in an unsigned int.  The
xenbus_file_read() function returns type ssize_t so the negative value
is returned as a positive value to the user.

This change forces another change to the min() macro.  Originally, the
min() macro used "unsigned" type which checkpatch complains about.  Also
unsigned type would break if "len" were not capped at MAX_RW_COUNT.  Use
size_t for the min().  (No effect on runtime for the min_t() change).

Fixes: 2fb3683e ("xen: Add xenbus device driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Link: https://lore.kernel.org/r/YutxJUaUYRG/VLVc@kiliSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/xenbus/xenbus_dev_frontend.c

@@ -128,7 +128,7 @@ static ssize_t xenbus_file_read(struct file *filp,
 {
 	struct xenbus_file_priv *u = filp->private_data;
 	struct read_buffer *rb;
-	unsigned i;
+	ssize_t i;
 	int ret;
 
 	mutex_lock(&u->reply_mutex);
@@ -148,7 +148,7 @@ static ssize_t xenbus_file_read(struct file *filp,
 	rb = list_entry(u->read_buffers.next, struct read_buffer, list);
 	i = 0;
 	while (i < len) {
-		unsigned sz = min((unsigned)len - i, rb->len - rb->cons);
+		size_t sz = min_t(size_t, len - i, rb->len - rb->cons);
 
 		ret = copy_to_user(ubuf + i, &rb->msg[rb->cons], sz);