crypto: strengthen the check for key slots overlapping with LUKS header

crypto: strengthen the check for key slots overlapping with LUKS header

The LUKS header data on disk is a fixed size, however, there's expected
to be a gap between the end of the header and the first key slot to get
alignment with the 2nd sector on 4k drives. This wasn't originally part
of the LUKS spec, but was always part of the reference implementation,
so it is worth validating this.

Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>