zvol_remove_minors_impl: remove all async fallbacks

Since both ZFS- and OS-sides of a zvol now take care of their own
locking and don't get in each other's way, there's no need for the very
complicated removal code to fall back to async tasks if the locks needed
at each stage can't be obtained right now.

Here we change it to be a linear three-step process: select zvols of
interest and flag them for removal, then wait for them to shed activity
and then remove them, and finally, free them.

Sponsored-by: Klara, Inc.
Sponsored-by: Railway Corporation
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Fedor Uporov <fuporov.vstack@gmail.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17625

include/sys/zvol_impl.h

@@ -56,6 +56,7 @@ typedef struct zvol_state {
 	atomic_t		zv_suspend_ref;	/* refcount for suspend */
 	krwlock_t		zv_suspend_lock;	/* suspend lock */
 	kcondvar_t		zv_removing_cv;	/* ready to remove minor */
+	list_node_t		zv_remove_node;	/* node on removal list */
 	struct zvol_state_os	*zv_zso;	/* private platform state */
 	boolean_t		zv_threading;	/* volthreading property */
 } zvol_state_t;