check that all ZFS snapshots within a pool are without holds

[grahamperrin@momh167-gjp4-hpelitebook8570p-freebsd] ~% sh
$ zfs get -Ht snapshot userrefs | grep -v $'\t'0 | cut -d $'\t' -f 1 | tr '\n' '\0' | xargs -0 zfs holds
NAME                                       TAG                                      TIMESTAMP
hpelitebook8570p/an example@20171017-1331  Yeah baby, I see you shaking that class  Sat Oct  7 13:33 2017
$ date ; uname -v
Sun 29 Oct 2017 18:07:10 GMT
FreeBSD 12.0-CURRENT #0 r320869: Mon Jul 10 13:57:55 UTC 2017     [email protected]:/usr/obj/usr/src/sys/GENERIC
$