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/zfs-tests/include/libtest.shlib

@@ -1599,6 +1599,31 @@ function destroy_pool #pool
 	return 0
 }
 
+# Return 0 if created successfully; $? otherwise
+#
+# $1 - dataset name
+# $2-n - dataset options
+
+function create_dataset #dataset dataset_options
+{
+	typeset dataset=$1
+
+	shift
+
+	if [[ -z $dataset ]]; then
+		log_note "Missing dataset name."
+		return 1
+	fi
+
+	if datasetexists $dataset ; then
+		destroy_dataset $dataset
+	fi
+
+	log_must zfs create $@ $dataset
+
+	return 0
+}
+
 # Return 0 if destroy successfully or the dataset exists; $? otherwise
 # Note: In local zones, this function should return 0 silently.
 #