Previosuly we did that only if the snapshot was mounted earlier, its
root vnode got recycled and then we accessed it again.
We never cleared the flag for a freshly mounted snapshot.
That was very inconsistent and probably a source of some bugs.
Or maybe that painted over some bugs which might get revealed now.
We should consistently clear the flag because we try very hard to
pretend that snapshots auto-mounted under .zfs are part of their
original filesystem. In other words, we try to hide the fact that they
are different filesystems / mountpoints.
MFC after: 5 weeks
cb68fd3513