Disable write merging on ZVOLs

Disable write merging on ZVOLs

The current ZVOL implementation does not explicitly set merge
options on ZVOL device queues, which results in the default merge
behavior.

Explicitly set QUEUE_FLAG_NOMERGES on ZVOL queues allowing the
ZIO pipeline to do its work.

Initial benchmarks (tiotest with no O_DIRECT) show random write
performance going up almost 3X on 8K ZVOLs, even after significant
rewrites of the logical space allocation.

Reviewed-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: RageLtMan <rageltman@sempervictus>
Issue #5902