ZVOL: Unify zvol minors operations and improve error handling

Now zvol minors creation logic is passed thru spa_zvol_taskq, like it
is doing for remove/rename zvol minors functions. Appropriate
zvol minors creation functions are refactored:
- The zvol_create_minor()/zvol_minors_create_recursive() were removed.
- The single zvol_create_minors() is added instead.

Also, it become possible to collect zvol minors subtasks status, to
detect, if some zvol minor subtask is failed in the subtasks chain.
The appropriate message is reported to zfs_dbgmsg buffer in this case.

Sponsored-by: vStack, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Fedor Uporov <fuporov.vstack@gmail.com>
Closes #17575

lib/libzpool/kernel.c

@@ -1031,13 +1031,7 @@ kmem_cache_reap_active(void)
 }
 
 void
-zvol_create_minor(const char *name)
-{
-	(void) name;
-}
-
-void
-zvol_create_minors_recursive(const char *name)
+zvol_create_minors(const char *name)
 {
 	(void) name;
 }