If during allocation of compressed clusters the cluster was already allocated
uncompressed, fail and properly release the l2_table (the latter avoids a
failed assertion).

While at it, make it return some real error numbers instead of -1.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NDong Xu Wang <wdongxu@linux.vnet.ibm.com>