ZVOL: Make zvol_volmode module parameter platform-independent

The module parameter name was not changed in FreeBSD sysctls
list: 'vfs.zfs.vol.mode'. Also, on Linux side the name is:
/sys/module/zfs/parameters/zvol_volmode.

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 #17386