bcachefs: Erasure coding: Track open stripes

bcachefs: Erasure coding: Track open stripes

This adds a new hash table for stripes being created or updated, instead
of hackily relying on the stripes heap.

This lets us reserve the slot for the new stripe up front, at the same
time as we would pick an existing stripe - if we were updating an
existing stripe - making the overall code more consistent.

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