hv: hv_balloon: avoid memory leak on alloc_error of 2MB memory block

If num_ballooned is not 0, we shouldn't neglect the
already-partially-allocated 2MB memory block(s).

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>