ZVOL: Make zvol_inhibit_dev module parameter platform-independent

The module parameter now is represented in FreeBSD sysctls list with
name: 'vfs.zfs.vol.inhibit_dev'. The default value is '0', same as on
Linux side.

Sponsored-by: vStack, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Signed-off-by: Fedor Uporov <fuporov.vstack@gmail.com>
Closes #17384