qcow2: Add subcluster support to zero_in_l2_slice()

qcow2: Add subcluster support to zero_in_l2_slice()

The QCOW_OFLAG_ZERO bit that indicates that a cluster reads as
zeroes is only used in standard L2 entries. Extended L2 entries use
individual 'all zeroes' bits for each subcluster.

This must be taken into account when updating the L2 entry and also
when deciding that an existing entry does not need to be updated.

Signed-off-by: Alberto Garcia <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-Id: <b61d61606d8c9b367bd641ab37351ddb9172799a.1594396418[email protected]>
Signed-off-by: Max Reitz <[email protected]>