libblkid: properly reset chain in probe_do_wipe()

libblkid: properly reset chain in probe_do_wipe()

The command wipefs is not able to remove all signatures from the
device if the first detected signature is linux raid. The another
superblock signatures are ignored. For example:

  # pvcreate /dev/sdb1
  # mdadm --create md8 --metadata=0.90 --chunk=64 --level=0 \
        --raid-devices=2 /dev/sdb1 /dev/sdb2

  # wipefs -a /dev/sdb1

will delete the linux raid signature only.

Reported-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>