block/qcow2: Lock s->lock in preallocate()

block/qcow2: Lock s->lock in preallocate()

preallocate() is and will be called only from places that do not
otherwise need to lock s->lock: Currently that is qcow2_create2(), as of
a future patch it will be called from qcow2_truncate(), too.

It therefore makes sense to move locking that mutex into preallocate()
itself.

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: 20170613202107[email protected]
Signed-off-by: Max Reitz <[email protected]>