memory: unref the memory region in simplify flatview

The memory region reference is increased when insert a range
into flatview range array, then decreased by destroy flatview.
If some flat range merged by flatview_simplify, the memory region
reference can not be decreased by destroy flatview any more.

In this case, start virtual machine by the command line:
qemu-system-x86_64
-name guest=ubuntu,debug-threads=on
-machine pc,accel=kvm,usb=off,dump-guest-core=off
-cpu host
-m 16384
-realtime mlock=off
-smp 8,sockets=2,cores=4,threads=1
-object memory-backend-file,id=ram-node0,prealloc=yes,mem-path=/dev/hugepages,share=yes,size=8589934592
-numa node,nodeid=0,cpus=0-3,memdev=ram-node0
-object memory-backend-file,id=ram-node1,prealloc=yes,mem-path=/dev/hugepages,share=yes,size=8589934592
-numa node,nodeid=1,cpus=4-7,memdev=ram-node1
-no-user-config
-nodefaults
-rtc base=utc
-no-shutdown
-boot strict=on
-device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2
-device virtio-scsi-pci,id=scsi0,bus=pci.0,addr=0x2
-device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x3
-drive file=ubuntu.qcow2,format=qcow2,if=none,id=drive-virtio-disk0,cache=none,aio=native
-device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1
-chardev pty,id=charserial0
-device isa-serial,chardev=charserial0,id=serial0
-device usb-tablet,id=input0,bus=usb.0,port=1
-vnc 0.0.0.0:0
-device VGA,id=video0,vgamem_mb=16,bus=pci.0,addr=0x5
-device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6
-msg timestamp=on

And run the script in guest OS:
while true
do
    setpci -s 00:06.0 04.b=03
	setpci -s 00:06.0 04.b=07
done

I found the reference of node0 HostMemoryBackendFile is a big one.
(gdb) p numa_info[0]->node_memdev->parent.ref
$6 = 1636278
(gdb)

Signed-off-by: King Wang<king.wang@huawei.com>
Message-Id: <20190712065241.11784-1-king.wang@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

memory.c

@@ -321,7 +321,7 @@ static bool can_merge(FlatRange *r1, FlatRange *r2)
 /* Attempt to simplify a view by merging adjacent ranges */
 static void flatview_simplify(FlatView *view)
 {
-    unsigned i, j;
+    unsigned i, j, k;
 
     i = 0;
     while (i < view->nr) {
@@ -332,6 +332,9 @@ static void flatview_simplify(FlatView *view)
             ++j;
         }
         ++i;
+        for (k = i; k < j; k++) {
+            memory_region_unref(view->ranges[k].mr);
+        }
         memmove(&view->ranges[i], &view->ranges[j],
                 (view->nr - j) * sizeof(view->ranges[j]));
         view->nr -= j - i;