bcachefs: Put erasure coding behind an EXPERIMENTAL kconfig option

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>