block: fix sector comparism in multiwrite_req_compare

The difference between the start sectors of two requests can be larger
than the size of the "int" type, which can lead to a not correctly
sorted multiwrite array and thus spurious I/O errors and filesystem
corruption due to incorrect request merges.

So instead of doing the cute sector arithmetics trick spell out the
exact comparisms.

Spotted by Kevin Wolf based on a testcase from Michael Tokarev.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -1929,7 +1929,19 @@ static void multiwrite_cb(void *opaque, int ret)
 
 static int multiwrite_req_compare(const void *a, const void *b)
 {
-    return (((BlockRequest*) a)->sector - ((BlockRequest*) b)->sector);
+    const BlockRequest *req1 = a, *req2 = b;
+
+    /*
+     * Note that we can't simply subtract req2->sector from req1->sector
+     * here as that could overflow the return value.
+     */
+    if (req1->sector > req2->sector) {
+        return 1;
+    } else if (req1->sector < req2->sector) {
+        return -1;
+    } else {
+        return 0;
+    }
 }
 
 /*