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

lib/libzpool/kernel.c

@@ -20,6 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2016 Actifio, Inc. All rights reserved.
  */
 
 #include <assert.h>
@@ -1354,3 +1355,24 @@ spl_fstrans_check(void)
 {
 	return (0);
 }
+
+void
+zvol_create_minors(spa_t *spa, const char *name, boolean_t async)
+{
+}
+
+void
+zvol_remove_minor(spa_t *spa, const char *name, boolean_t async)
+{
+}
+
+void
+zvol_remove_minors(spa_t *spa, const char *name, boolean_t async)
+{
+}
+
+void
+zvol_rename_minors(spa_t *spa, const char *oldname, const char *newname,
+    boolean_t async)
+{
+}