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

module/zfs/dmu_objset.c

@@ -26,6 +26,7 @@
  * Copyright (c) 2014 Spectra Logic Corporation, All rights reserved.
  * Copyright (c) 2015 Nexenta Systems, Inc. All rights reserved.
  * Copyright (c) 2015, STRATO AG, Inc. All rights reserved.
+ * Copyright (c) 2016 Actifio, Inc. All rights reserved.
  */
 
 /* Portions Copyright 2010 Robert Milkowski */
@@ -868,6 +869,8 @@ dmu_objset_create_sync(void *arg, dmu_tx_t *tx)
 	}
 
 	spa_history_log_internal_ds(ds, "create", tx, "");
+	zvol_create_minors(dp->dp_spa, doca->doca_name, B_TRUE);
+
 	dsl_dataset_rele(ds, FTAG);
 	dsl_dir_rele(pdd, FTAG);
 }
@@ -961,6 +964,7 @@ dmu_objset_clone_sync(void *arg, dmu_tx_t *tx)
 	dsl_dataset_name(origin, namebuf);
 	spa_history_log_internal_ds(ds, "clone", tx,
 	    "origin=%s (%llu)", namebuf, origin->ds_object);
+	zvol_create_minors(dp->dp_spa, doca->doca_clone, B_TRUE);
 	dsl_dataset_rele(ds, FTAG);
 	dsl_dataset_rele(origin, FTAG);
 	dsl_dir_rele(pdd, FTAG);