mbroz and karelzak libblkid: Check for a secondary LUKS2 header.
This patch adds search for a secondary LUKS2 header,
if the primary one is corrupted.

This patch is primarily needed for wipefs that should wipe
both signatures (otherwise LUKS2 header recovery can use
secondary header and revert wipefs action).

Signed-off-by: Milan Broz <gmazyland@gmail.com>
Latest commit 8bee1a2 Jul 11, 2018