feature request: Make "zfs set snapdir=visible tank/test" work.

[hboetes]

I wanted to have all snapshots mounted, so I set zfs set snapdir=visible tank/test only to notice that zfs-auto-snapshot can't delete them anymore since the snapshots are mounted and it doesn't know how to unmount them.

This begs for a feature of course. :)

[hboetes]

@kpande well it's still available in the debian repos, so for anyone running into this issue:

I just found the fix:
replace the destroy command:
do_run "zfs destroy $FLAGS '$jj'"
with
do_run "zfs destroy '$jj'"