dm: restrict dm device size to 2^63-512 bytes

The devices with size >= 2^63 bytes can't be used reliably by userspace
because the type off_t is a signed 64-bit integer.

Therefore, we limit the maximum size of a device mapper device to
2^63-512 bytes.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>