blkid: fix ZFS device detection

blkid: fix ZFS device detection

While the main blkid functionality is in util-linux, there is still
use for blkid on non-Linux platforms.

Fix the ZFS device detection by looking at multiple uberblocks to
see if any are present, rather than looking for the ZFS boot block
which is not always present.

There are slots for up to 128 uberblocks, but the first 4 are not
written to disk on a newly-formatted filesystem, so check several
of them at different offsets within the uberblock array.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>