Investigate automatically forcing xattr_fallback=on so it can be off by default

I know for sure there are cases where we can't know the optimal thing to do, such as when we've received a dataset without the property or when we've imported a pool without the property. There's no way I can think of to determine a priori which way the xattrs are named in these cases, and in fact there could be both formats present in the same dataset. Being able to read both by default ensures the user does not unwittingly lose their xattrs. xattr_fallback won't cause the xattr_compat feature to become active.

I'm not very familiar with the DSL part of ZFS yet, which is where I think I'll need to be working to figure out if we can safely disable fallback automatically, or rather, to be able to know when we need to force xattr_fallback=on so we can change the default to xattr_fallback=off. I'm still getting up to speed in this area.