bcachefs: Kill some unneeded references to c->flags

This drops some unneeded references to JOURNAL_REPLAY_DONE in c->flags:
we're already mirroring it in btree_trans, we just weren't using it
consistently.

We may want to do this with more flags:
  btree_iter.c:   unsigned nr = test_bit(BCH_FS_STARTED, &c->flags)
  btree_update_leaf.c:    if (unlikely(!test_bit(BCH_FS_MAY_GO_RW, &c->flags))) {
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_update_leaf.c

@@ -615,7 +615,7 @@ bch2_trans_commit_write_locked(struct btree_trans *trans,
 		 */
 		i->old_v = bch2_btree_path_peek_slot(i->path, &i->old_k).v;
 
-		if (unlikely(!test_bit(JOURNAL_REPLAY_DONE, &c->journal.flags))) {
+		if (unlikely(trans->journal_replay_not_finished)) {
 			struct bkey_i *j_k =
 				bch2_journal_keys_peek_slot(c, i->btree_id, i->level,
 							    i->k->k.p);