qcow2: Fix alloc_cluster_abort() for pre-existing clusters

qcow2: Fix alloc_cluster_abort() for pre-existing clusters

handle_alloc() reuses preallocated zero clusters.  If anything goes
wrong during the data write, we do not change their L2 entry, so we
must not let qcow2_alloc_cluster_abort() free them.

Fixes: 8b24cd141549b5b264baeddd4e72902cfb5de23b
Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200225143130.111267-2-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 3ede935fdbbd5f7b24b4724bbfb8938acb5956d8)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>