ZVOL: Implement zvol_alloc() function on FreeBSD side

Implement zvol_alloc() function on FreeBSD side to increase code base
compatibility with Linux. Also, fix issue with late returning in case
if volmode=none.

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 #17482