qcow2: Pull check_refblocks() up

Pull check_refblocks() before calculate_refcounts() so we can drop its
static declaration.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
