release: Add missing /var/crash ZFS dataset

This matches the layout from bsdinstall.

MFC after: 1 week
Fixes: 89585511cc05 ("release: Add support for creating ZFS-based VM images")
Pull Request: https://github.com/freebsd/freebsd-src/pull/981

(cherry picked from commit adc215de9cabadb89eb4a1c99e047906eacaa58a)