libblkid: Check offset in LUKS2 header

libblkid: Check offset in LUKS2 header

LUKS2 binary header contains offset field that describes where
the header should be located.

If this offset is not correct, blkid should tread this header
as invalid.

This patch fixes problem when both swap and LUKS headers are
present (LUKS header was swapped out) and detected LUKS header
is at a wrong offset.
As LUKS has higher priority, it confuses detection.

Signed-off-by: Milan Broz <gmazyland@gmail.com>