bcachefs: Improve trans->extra_journal_entries

Instead of using a darray, we now allocate journal entries for the
transaction commit path with our normal bump allocator - with an inlined
fastpath, and using btree_transaction_stats to remember how much to
initially allocate so as to avoid transaction restarts.

This is prep work for converting write buffer updates to use this
mechanism.

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