bcachefs: Data update path no longer leaves cached replicas

It turns out that it's currently impossible to invalidate buckets
containing only cached data if they're part of a stripe. The normal
bucket invalidate path can't do it because we have to be able to
incerement the bucket's gen, which isn't correct becasue it's still a
member of the stripe - and the bucket invalidate path makes the bucket
availabel for reuse right away, which also isn't correct for buckets in
stripes.

What would work is invalidating cached data by following backpointers,
except that cached replicas don't currently get backpointers - because
they would be awkward for the existing bucket invalidate path to delete
and they haven't been needed elsewhere.

So for the time being, to prevent running out of space in stripes,
switch the data update path to not leave cached replicas; we may revisit
this in the future.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

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