qcow2: Always use error path in l2_allocate

Just returning -errno in some cases prevents
trace_qcow2_l2_allocate_done from being executed (and, in one case, also
the unused allocated L2 table from being freed). Always going down the
error path fixes this.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
