howto/ganeti: add additional net-custom gnt-dal settings

verify-alg allows to trigger an on-line verify to check to out-of-sync
blocks between primary and secondary

max-buffers increases the ammount of memory allocated per volume which
improves sync rates significantly

howto/ganeti.md

@@ -3142,6 +3142,7 @@ gnt-cluster modify -H kvm:migration_downtime=500
 gnt-cluster modify -H kvm:migration_caps=postcopy-ram
 gnt-cluster modify -H kvm:cpu_type=host
 gnt-cluster modify -D drbd:c-plan-ahead=0,disk-custom='--c-plan-ahead 0'
+gnt-cluster modify -D drbd:net-custom='--verify-alg sha1 --max-buffers 8k'
 gnt-cluster modify --uid-pool 4000-4019
 ```