FreeBSD: Define crgetzoneid appropriately

The previous ZFS implementation on FreeBSD had ifdefs to use jailed()
instead of crgetzoneid() in dsl_dir.c, however we can simply provide an
appropriate definition of crgetzoneid for the same effect.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #10851