qcow2: Fix corruption after refblock allocation

qcow2: Fix corruption after refblock allocation

Refblock allocation code needs to take into consideration that update_refcount
will load a different refcount block into the cache, so it must initialize the
cache for a new refcount block only afterwards. Not doing this means that not
only the refcount in the wrong block is updated, but also that the caller will
work on the wrong block.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 25408c09502be036e5575754fe54019ed4ed5dfa)