Fix ZVOL_DIR

We only use ZVOL_DIR on FreeBSD, and on FreeBSD it isn't correct.

Move the definition to the file where it is needed, and define it as
/dev/zvol/.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10337