Add support for asynchronous zvol minor operations

zfsonlinux issue #2217 - zvol minor operations: check snapdev
property before traversing snapshots of a dataset

zfsonlinux issue #3681 - lock order inversion between zvol_open()
and dsl_pool_sync()...zvol_rename_minors()

Create a per-pool zvol taskq for asynchronous zvol tasks.
There are a few key design decisions to be aware of.

* Each taskq must be single threaded to ensure tasks are always
  processed in the order in which they were dispatched.

* There is a taskq per-pool in order to keep the pools independent.
  This way if one pool is suspended it will not impact another.

* The preferred location to dispatch a zvol minor task is a sync
  task.  In this context there is easy access to the spa_t and
  minimal error handling is required because the sync task must
  succeed.

Support for asynchronous zvol minor operations address issue #3681.

Signed-off-by: Boris Protopopov <boris.protopopov@actifio.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2217
Closes #3678
Closes #3681

include/sys/spa_impl.h

@@ -23,6 +23,7 @@
  * Copyright (c) 2011, 2015 by Delphix. All rights reserved.
  * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
  * Copyright (c) 2014 Spectra Logic Corporation, All rights reserved.
+ * Copyright (c) 2016 Actifio, Inc. All rights reserved.
  */
 
 #ifndef _SYS_SPA_IMPL_H
@@ -253,6 +254,7 @@ struct spa {
 	uint64_t	spa_errata;		/* errata issues detected */
 	spa_stats_t	spa_stats;		/* assorted spa statistics */
 	hrtime_t	spa_ccw_fail_time;	/* Conf cache write fail time */
+	taskq_t		*spa_zvol_taskq;	/* Taskq for minor managment */
 
 	/*
 	 * spa_refcount & spa_config_lock must be the last elements