qcow2: fix #ifdef'd qcow2_check_refcounts() callers

The DEBUG_ALLOC qcow2.h macro enables additional consistency checks
throughout the code.  This makes it easier to spot corruptions that are
introduced during development.  Since consistency check is an expensive
operation the DEBUG_ALLOC macro is used to compile checks out in normal
builds and qcow2_check_refcounts() calls missed the addition of a new
function argument.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
