qcow2: keep reference on zeroize with discard-no-unref enabled

When the discard-no-unref flag is enabled, we keep the reference for
normal discard requests.
But when a discard is executed on a snapshot/qcow2 image with backing,
the discards are saved as zero clusters in the snapshot image.

When committing the snapshot to the backing file, not
discard_in_l2_slice is called but zero_in_l2_slice. Which did not had
any logic to keep the reference when discard-no-unref is enabled.

Therefor we add logic in the zero_in_l2_slice call to keep the reference
on commit.

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1621
Signed-off-by: Jean-Louis Dupond <jean-louis@dupond.be>
Message-Id: <20231003125236.216473-2-jean-louis@dupond.be>
[hreitz: Made the documentation change more verbose, as discussed
         on-list]
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>

qapi/block-core.json

@@ -3528,16 +3528,20 @@
 # @pass-discard-other: whether discard requests for the data source
 #     should be issued on other occasions where a cluster gets freed
 #
-# @discard-no-unref: when enabled, discards from the guest will not
-#     cause cluster allocations to be relinquished.  This prevents
-#     qcow2 fragmentation that would be caused by such discards.
-#     Besides potential performance degradation, such fragmentation
-#     can lead to increased allocation of clusters past the end of the
-#     image file, resulting in image files whose file length can grow
-#     much larger than their guest disk size would suggest.  If image
-#     file length is of concern (e.g. when storing qcow2 images
-#     directly on block devices), you should consider enabling this
-#     option.  (since 8.1)
+# @discard-no-unref: when enabled, data clusters will remain
+#     preallocated when they are no longer used, e.g. because they are
+#     discarded or converted to zero clusters.  As usual, whether the
+#     old data is discarded or kept on the protocol level (i.e. in the
+#     image file) depends on the setting of the pass-discard-request
+#     option.  Keeping the clusters preallocated prevents qcow2
+#     fragmentation that would otherwise be caused by freeing and
+#     re-allocating them later.  Besides potential performance
+#     degradation, such fragmentation can lead to increased allocation
+#     of clusters past the end of the image file, resulting in image
+#     files whose file length can grow much larger than their guest disk
+#     size would suggest.  If image file length is of concern (e.g. when
+#     storing qcow2 images directly on block devices), you should
+#     consider enabling this option.  (since 8.1)
 #
 # @overlap-check: which overlap checks to perform for writes to the
 #     image, defaults to 'cached' (since 2.2)