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When destroying a zvol, it is not "unpublished" from the system (that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