Allow fetching the pool from the device at mount

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>
Return -EOPNOTSUPP for ZFS_IOC_{GET|SET}FLAGS

Until these hooks are fully implemented return the expected
-EOPNOTSUPP error to indicate they are not functional.  This
allows test suites such as xfstests to cleanly skip testing
this functionality until it's implemented.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #229
Fix module probe failure on 32-bit systems

Previous commit 7ef5e54e2e28884a04dc800657967b891239e933 caused
module probe failure on 32-bit systems, dmesg showed

  Unknown symbol __moddi3

This was caused by the modulo operation 'gethrtime() % tqs->stqs_count'
in the committed code.  Instead of implementing __moddi3 for all 32-bit
systems, Behlendorf advised we can just cast the return value of
gethrtime() into a uint64_t, since gethrtime does not return negative
value on all circumstances we need not care about the potential overflow.

Signed-off-by: Ying Zhu <casualfisher@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1551