To simplify integration with the xfstests test suite the
mount.zfs helper has been extended. When passed a block
device (/dev/sdX) to mount, instead of a pool/dataset,
the pool name will be read from any existing zfs label
and used. This allows you to mount the root dataset of
a zfs filesystem by specifing any of the member vdevs.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>