bcachefs: Refactor open_bucket code

bcachefs: Refactor open_bucket code

Prep work for adding a hash table of open buckets - instead of embedding
a bch_extent_ptr, we need to refer to the bucket directly so that we're
not calling sector_to_bucket() in the hash table lookup code, which has
an expensive divide.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>