Add crossmnt to list of default NFS options

This is needed on focal if a client is to be able to access snapshots over NFS.
From the docs I don't see why we didn't also need this option on xenial,
but empirically, we need it on focal. (e.g. RT-207229)

ROOT/DEBIAN/control

@@ -3,6 +3,6 @@ Priority: optional
 Section: otherosfs
 Maintainer: Chemistry COs <support@ch.cam.ac.uk>
 Architecture: all
-Version: 0.9-ch76
+Version: 0.9-ch77
 Depends: zfs-dkms, postgresql-13 | postgresql-9.5 | postgresql-9.4 , liblockfile-simple-perl, libdbi-perl, libjson-perl, libzfs-perl-chem, libnet-openssh-perl, libdbd-pg-perl, mbuffer, rsync, nfs-kernel-server, pv, libwww-curl-perl
 Description: a backup system using ZFS (repository 'backup-scheduler')

ROOT/usr/lib/chem-zfs-backup-server/new-backup-rsnapshot

@@ -137,7 +137,7 @@ function getsshkey() {
 HOSTID=`ensurehost $FQDN`
 echo Host ID is $HOSTID
 ensurezfs $TGT
-$ZFSCMD set sharenfs=ro=${FQDN},no_root_squash $TGT
+$ZFSCMD set sharenfs=ro=${FQDN},no_root_squash,crossmnt $TGT
 
 # Do root backup
 backupdir $FQDN $TGT/zfs-rsnap /