Document and subtlely change the zfs.mount_snapshot jail parameter

[jamie]

Summary

The parameter zfs.mount_snapshot was added that allows jailed users to mount ZFS snapshots. It also fixes bug 274263, which is about accessing those snapshots via the .zfs directory, not mounting them. Both of these uses should be documented in jail(8).

Also, the parameter operates as a boolean, but is listed as an plain integer (format "I" instead of "B"). Changing it to a proper boolean parameter would allow the existing boolean handling in jail.conf (i.e. "zfs.mount_snapshot" and "zfs.nomount_snapshot"), while the current "zfs.mount_snapshot=1" would still work.

The ZFS code change is trivial, but I'd still like buy-in because it's contrib code.

[avg]

I would subtly change the title :-)
Otherwise, looks good to me.

[zlei]

I guess the change for sys/contrib/openzfs/module/os/freebsd/zfs/zfs_vfsops.c go to upstream first. Will it ?

[jamie]

In D45647#1041512, @zlei wrote:

I guess the change for sys/contrib/openzfs/module/os/freebsd/zfs/zfs_vfsops.c go to upstream first. Will it ?

See, this is the kind of thing that keeps me from checking it in. But I'll sit a few days longer, and absent anyone saying "no, wait!" I'll let it get upstreamed whenever.