zfs initialize performance enhancements

PROBLEM
========

When invoking "zpool initialize" on a pool the command will
create a thread to initialize each disk. Unfortunately, it does
this serially across many transaction groups which can result
in commands taking a long time to return to the user and may
appear hung. The same thing is true when trying to suspend/cancel
the operation.

SOLUTION
=========

This change refactors the way we invoke the initialize interface
to ensure we can start or stop the intialization in just a few
transaction groups.

When stopping or cancelling a vdev initialization perform it
in two phases.  First signal each vdev initialization thread
that it should exit, then after all threads have been signaled
wait for them to exit.

On a pool with 40 leaf vdevs this reduces the vdev initialize
stop/cancel time from ~10 minutes to under a second.  The reason
for this is spa_vdev_initialize() no longer needs to wait on
multiple full TXGs per leaf vdev being stopped.

This commit additionally adds some missing checks for the passed
"initialize_vdevs" input nvlist.  The contents of the user provided
input "initialize_vdevs" nvlist must be validated to ensure all
values are uint64s.  This is done in zfs_ioc_pool_initialize() in
order to keep all of these checks in a single location.

Updated the innvl and outnvl comments to match the formatting used
for all other new sytle ioctls.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Wilson <george.wilson@delphix.com>
Closes #8230

cmd/ztest/ztest.c

@@ -6404,7 +6404,14 @@ ztest_initialize(ztest_ds_t *zd, uint64_t id)
 	spa_config_exit(spa, SCL_VDEV, FTAG);
 
 	uint64_t cmd = ztest_random(POOL_INITIALIZE_FUNCS);
-	error = spa_vdev_initialize(spa, guid, cmd);
+
+	nvlist_t *vdev_guids = fnvlist_alloc();
+	nvlist_t *vdev_errlist = fnvlist_alloc();
+	fnvlist_add_uint64(vdev_guids, path, guid);
+	error = spa_vdev_initialize(spa, vdev_guids, cmd, vdev_errlist);
+	fnvlist_free(vdev_guids);
+	fnvlist_free(vdev_errlist);
+
 	switch (cmd) {
 	case POOL_INITIALIZE_CANCEL:
 		if (ztest_opts.zo_verbose >= 4) {