ZVOL: Return early, if volmode is ZFS_VOLMODE_NONE on FreeBSD side

Return from zvol_os_create_minor() function immediately after
dsl_prop_get_integer() call if volmode property value is set to
'none', like it is doing on Linux side.

Sponsored-by: vStack, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Fedor Uporov <fuporov.vstack@gmail.com>
Closes #17405