bcachefs: Don't keep tons of cached pointers around

bcachefs: Don't keep tons of cached pointers around

We had a bug report where the data update path was creating an extent
that failed to validate because it had too many pointers; almost all of
them were cached.

To fix this, we have:
- want_cached_ptr(), a new helper that checks if we even want a cached
  pointer (is on appropriate target, device is readable).

- bch2_extent_set_ptr_cached() now only sets a pointer cached if we want
  it.

- bch2_extent_normalize_by_opts() now ensures that we only have a single
  cached pointer that we want.

While working on this, it was noticed that this doesn't work well with
reflinked data and per-file options. Another patch series is coming that
plumbs through additional io path options through bch_extent_rebalance,
with improved option handling.

Reported-by: Reed Riley <reed@riley.engineer>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>