block/iscsi: dont leave allocmap in an invalid state on UNMAP failure

we forgot to set the allocmap to invalid if an UNMAP call fails.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Message-Id: <1512733868-9009-2-git-send-email-pl@kamp.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
