zvol: remove the OS-side minor before freeing the zvol

When destroying a zvol, it is not "unpublished" from the system (that
is, /dev/zd* node removed) until zvol_os_free(). Under Linux, at the
time del_gendisk() and put_disk() are called, the device node may still
be have an active hold, from a userspace program or something inside the
kernel (a partition probe). As it is currently, this can lead to calls
to zvol_open() or zvol_release() while the zvol_state_t is partially or
fully freed. zvol_open() has some protection against this by checking
that private_data is NULL, but zvol_release does not.

This implements a better ordering for all of this by adding a new
OS-side method, zvol_os_remove_minor(), which is responsible for fully
decoupling the "private" (OS-side) objects from the zvol_state_t. For
Linux, that means calling put_disk(), nulling private_data, and freeing
zv_zso.

This takes the place of zvol_os_clear_private(), which was a nod in that
direction but did not do enough, and did not do it early enough.

Equivalent changes are made on the FreeBSD side to follow the API
change.

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

@@ -20,7 +20,7 @@
  * CDDL HEADER END
  */
 /*
- * Copyright (c) 2024, Klara, Inc.
+ * Copyright (c) 2024, 2025, Klara, Inc.
  */
 
 #ifndef	_SYS_ZVOL_IMPL_H
@@ -135,7 +135,7 @@ int zvol_os_rename_minor(zvol_state_t *zv, const char *newname);
 int zvol_os_create_minor(const char *name);
 int zvol_os_update_volsize(zvol_state_t *zv, uint64_t volsize);
 boolean_t zvol_os_is_zvol(const char *path);
-void zvol_os_clear_private(zvol_state_t *zv);
+void zvol_os_remove_minor(zvol_state_t *zv);
 void zvol_os_set_disk_ro(zvol_state_t *zv, int flags);
 void zvol_os_set_capacity(zvol_state_t *zv, uint64_t capacity);