qcow2: drop flush in update_cluster_refcount()

qcow2: drop flush in update_cluster_refcount()

The update_cluster_refcount() function increments/decrements a cluster's
refcount and then returns the new refcount value.

There is no need to flush since both update_cluster_refcount() callers
already take care of this:

1. qcow2_alloc_bytes() calls update_cluster_refcount() when compressed
   sectors will be appended to an existing cluster with enough free
   space.  qcow2_alloc_bytes() already flushes so there is no need to do
   so in update_cluster_refcount().

2. qcow2_update_snapshot_refcount() sets a cache dependency on refcounts
   if it needs to update L2 entries.  It also flushes before completing.

Removing this flush significantly speeds up qcow2 snapshot creation:

  $ qemu-img create -f qcow2 test.qcow2 -o size=50G,preallocation=metadata
  $ time qemu-img snapshot -c new test.qcow2

Time drops from more than 3 minutes to under 1 second.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>