bcachefs: Kill some unneeded references to c->flags

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>