bcachefs: Normal update/commit path now works before going RW

bcachefs: Normal update/commit path now works before going RW

This improves __bch2_trans_commit - early in the recovery process, when
we're running btree_gc and before we want to go RW, it now uses
bch2_journal_key_insert() to add the update to the list of updates for
journal replay to do, instead of btree_gc having to use separate
interfaces depending on whether we're running at bringup or, later,
runtime.

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