qcow2: qcow2_co_preadv: improve locking

qcow2: qcow2_co_preadv: improve locking

Background: decryption will be done in threads, to take benefit of it,
we should move it out of the lock first.

But let's go further: it turns out, that only
qcow2_get_cluster_offset() needs locking, so reduce locking to it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-id: 20190506142741[email protected]
Reviewed-by: Alberto Garcia <[email protected]>
Signed-off-by: Max Reitz <[email protected]>