Add spa _os() hooks

Add hooks for when spa is created, exported, activated and
deactivated. Used by macOS to attach iokit, and lock
kext as busy (to stop unloads).

Userland, Linux, and, FreeBSD have empty stubs.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #12801

module/os/freebsd/zfs/spa_os.c

@@ -268,3 +268,27 @@ spa_history_zone(void)
 {
 	return ("freebsd");
 }
+
+void
+spa_import_os(spa_t *spa)
+{
+	(void) spa;
+}
+
+void
+spa_export_os(spa_t *spa)
+{
+	(void) spa;
+}
+
+void
+spa_activate_os(spa_t *spa)
+{
+	(void) spa;
+}
+
+void
+spa_deactivate_os(spa_t *spa)
+{
+	(void) spa;
+}

module/os/linux/zfs/spa_misc_os.c

@@ -108,3 +108,27 @@ spa_history_zone(void)
 {
 	return ("linux");
 }
+
+void
+spa_import_os(spa_t *spa)
+{
+	(void) spa;
+}
+
+void
+spa_export_os(spa_t *spa)
+{
+	(void) spa;
+}
+
+void
+spa_activate_os(spa_t *spa)
+{
+	(void) spa;
+}
+
+void
+spa_deactivate_os(spa_t *spa)
+{
+	(void) spa;
+}

module/zfs/spa.c

@@ -1315,6 +1315,8 @@ spa_activate(spa_t *spa, spa_mode_t mode)
 	    spa_error_entry_compare, sizeof (spa_error_entry_t),
 	    offsetof(spa_error_entry_t, se_avl));
 
+	spa_activate_os(spa);
+
 	spa_keystore_init(&spa->spa_keystore);
 
 	/*
@@ -1451,6 +1453,9 @@ spa_deactivate(spa_t *spa)
 		thread_join(spa->spa_did);
 		spa->spa_did = 0;
 	}
+
+	spa_deactivate_os(spa);
+
 }
 
 /*
@@ -6031,6 +6036,8 @@ spa_create(const char *pool, nvlist_t *nvroot, nvlist_t *props,
 	spa->spa_minref = zfs_refcount_count(&spa->spa_refcount);
 	spa->spa_load_state = SPA_LOAD_NONE;
 
+	spa_import_os(spa);
+
 	mutex_exit(&spa_namespace_lock);
 
 	return (0);
@@ -6214,6 +6221,8 @@ spa_import(char *pool, nvlist_t *config, nvlist_t *props, uint64_t flags)
 
 	zvol_create_minors_recursive(pool);
 
+	spa_import_os(spa);
+
 	return (0);
 }
 
@@ -6461,6 +6470,8 @@ spa_export_common(const char *pool, int new_state, nvlist_t **oldconfig,
 	}
 
 export_spa:
+	spa_export_os(spa);
+
 	if (new_state == POOL_STATE_DESTROYED)
 		spa_event_notify(spa, NULL, NULL, ESC_ZFS_POOL_DESTROY);
 	else if (new_state == POOL_STATE_EXPORTED)