Update pam_zfs_key.c defaultt path for FreeBSD

As described in https://github.com/freebsd/freebsd-src/pull/1305,
FreeBSD's installer defaults to zroot/home for user home directories.

For FreeBSD only, set the default prefix for pam_zfs_key to match.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Eric A. Borisch <eborisch@gmail.com>
Closes #17600