qcow2: fix leak of Qcow2DiscardRegion in update_refcount_discard

When the Qcow2DiscardRegion is adjacent to another one referenced by "d",
free this Qcow2DiscardRegion metadata referenced by "p" after
it was removed from s->discards queue.

Signed-off-by: Zhang Haoyu <zhanghy@sangfor.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
