libblkid: fix ZFS superblok offset for SBMAGIC

 # wipefs /dev/sdd1

 offset               type
 ----------------------------------------------------------------
 0x65000              zfs_member   [raid]
                      LABEL: zm1
                      UUID:  18067835215330778964

 and "wipefs -o 0x65000 /dev/sdd1" seems to run in an endless loop.

Reported-by: Helmut Hullen <Hullen@t-online.de>
Signed-off-by: Karel Zak <kzak@redhat.com>