bcachefs: Don't use in-memory bucket array for alloc updates

bcachefs: Don't use in-memory bucket array for alloc updates

More prep work for getting rid of the in-memory bucket array: now that
we have BTREE_ITER_WITH_JOURNAL, the allocator code can do ntree lookups
before journal replay is finished, and there's no longer any need for it
to get allocation information from the in-memory bucket array.

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