MFV r316917: 7968 multi-threaded spa_sync()

MFV r316917: 7968 multi-threaded spa_sync()

illumos/illumos-gate@94c2d0eb22e9624151ee84a7edbf7178e1bf4087
https://github.com/illumos/illumos-gate/commit/94c2d0eb22e9624151ee84a7edbf7178e1bf4087

https://www.illumos.org/issues/7968
  spa_sync() iterates over all the dirty dnodes and processes each of them by
  calling dnode_sync(). If there are many dirty dnodes (e.g. because we created
  or removed a lot of files), the single thread of spa_sync() calling
  dnode_sync() can become a bottleneck. Additionally, if many dnodes are dirtied
  concurrently in open context (e.g. due to concurrent file creation), the
  os_lock will experience lock contention via dnode_setdirty().
  The solution is to track dirty dnodes on a multilist_t, and for spa_sync() to
  use separate threads to process each of the sublists in the multilist.
  On the concurrent file creation microbenchmark, the performance improvement
  from dnode_setdirty() is up to 7%. Additionally, the wall clock time spent in
  spa_sync() is reduced to 15%-40% of the single-threaded case. In terms of cost/
  reward, once the other bottlenecks are addressed, fixing this bug will provide
  a medium-large performance gain and require a medium amount of effort to
  implement.

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Matthew Ahrens <mahrens@delphix.com>
MFC after:	3 weeks