qcow2-refcount: don't mask corruptions under internal errors

qcow2-refcount: don't mask corruptions under internal errors

No reasons for not reporting found corruptions as corruptions in case
of some internal errors, especially in case of just failed to fix l2
entry (and in this case, missed corruptions may influence comparing
logic, when we calculate difference between corruptions fields of two
results)

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-id: 20190227131433[email protected]
Reviewed-by: Max Reitz <[email protected]>
Signed-off-by: Max Reitz <[email protected]>