virtio-mem: Fix the bitmap index of the section offset

vmem->bitmap indexes the memory region of the virtio-mem backend at a
granularity of block_size. To calculate the index of target section offset,
the block_size should be divided instead of the bitmap_size.

Fixes: 2044969 ("virtio-mem: Implement RamDiscardManager interface")
Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Message-Id: <20221216062231.11181-1-chenyi.qiang@intel.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: David Hildenbrand <david@redhat.com>
(cherry picked from commit b11cf32)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/virtio/virtio-mem.c

@@ -235,7 +235,7 @@ static int virtio_mem_for_each_plugged_section(const VirtIOMEM *vmem,
     uint64_t offset, size;
     int ret = 0;
 
-    first_bit = s->offset_within_region / vmem->bitmap_size;
+    first_bit = s->offset_within_region / vmem->block_size;
     first_bit = find_next_bit(vmem->bitmap, vmem->bitmap_size, first_bit);
     while (first_bit < vmem->bitmap_size) {
         MemoryRegionSection tmp = *s;
@@ -267,7 +267,7 @@ static int virtio_mem_for_each_unplugged_section(const VirtIOMEM *vmem,
     uint64_t offset, size;
     int ret = 0;
 
-    first_bit = s->offset_within_region / vmem->bitmap_size;
+    first_bit = s->offset_within_region / vmem->block_size;
     first_bit = find_next_zero_bit(vmem->bitmap, vmem->bitmap_size, first_bit);
     while (first_bit < vmem->bitmap_size) {
         MemoryRegionSection tmp = *s;