xfs_io: add a quiet option to bulkstat

xfs_io: add a quiet option to bulkstat

This is purely for driving the kernel bulkstat operations as hard
as userspace can drive them - we don't care about the actual output,
just want to drive maximum IO rates through the inode cache.

Bulkstat at 3.4 million inodes a second via xfs_io currently burns
about 30% of CPU time just formatting and outputting the stat
information to stdout and dumping it to /dev/null.

		wall time	rate	IOPS	bandwidth
unpatched	17.823s		3.4M/s	70k	1.9GB/s
with -q		15.682		6.1M/s  150k	3.5GB/s

The disks are at about 30% of max bandwidth and only at 70kiops, so
this CPU can be used to drive the kernel and IO subsystem harder.

Wall time doesn't really go down on this specific test because the
increase in inode cache turn-over (about 10GB/s of cached metadata
(in-core inodes and buffers) is being cycled through memory on a
machine with 16GB of RAM) and that hammers memory reclaim into a
utter mess that often takes seconds for it to recover from...

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>