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 may be 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.
[kzak@redhat.com: - port e2fsprogs patch to util-linux-ng]
Signed-off-by: Andreas Dilger <adilger@sun.com> Signed-off-by: Karel Zak <kzak@redhat.com>