We still have disk space accounting changes coming for erasure coding,
and the changes won't be as strictly backwards compatible as they'd
ought to be - specifically, we need to start accounting striped data
under a separate counter in bch_alloc (which describes buckets).

A fsck will suffice for upgrading/downgrading, but since erasure coding
is the most incomplete major feature of bcachefs it still makes sense to
put behind a separate kconfig option, so that users are fully aware.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>