l2_allocate has some intermediate states in which the image is inconsistent.
Change the order to write to the L1 table only after the new L2 table has
successfully been initialized.

Also reset the L2 cache in failure case, it's very likely wrong.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
(cherry picked from commit 175e1152)

Conflicts:

	block/qcow2-cluster.c
Signed-off-by: NKevin Wolf <kwolf@redhat.com>