OpenZFS 9082 - Add ZFS performance test targeting ZIL latency

This adds a new test to measure ZIL performance.

- Adds the ability to induce IO delays with zinject
- Adds a new variable (PERF_NTHREADS_PER_FS) to allow fio threads to
  be distributed to individual file systems as opposed to all IO going
  to one, as happens elsewhere.
- Refactoring of do_fio_run

Authored by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: John Wren Kennedy <jwk404@gmail.com>

OpenZFS-issue: https://www.illumos.org/issues/9082
OpenZFS-commit: https://github.com/openzfs/openzfs/pull/634
External-issue: DLPX-48625
Closes #7491

tests/runfiles/perf-regression.run

@@ -10,7 +10,7 @@
 #
 
 #
-# Copyright (c) 2015 by Delphix. All rights reserved.
+# Copyright (c) 2015, 2016 by Delphix. All rights reserved.
 #
 
 [DEFAULT]
@@ -27,6 +27,6 @@ tags = ['perf']
 [tests/perf/regression]
 tests = ['sequential_writes', 'sequential_reads', 'sequential_reads_arc_cached',
     'sequential_reads_arc_cached_clone', 'sequential_reads_dbuf_cached',
-    'random_reads', 'random_writes', 'random_readwrite']
+    'random_reads', 'random_writes', 'random_readwrite', 'random_writes_zil']
 post =
 tags = ['perf', 'regression']