bcachefs: readdir fixes

bcachefs: readdir fixes

- Don't call bch2_trans_relock() after dir_emit(); taking a transaction
  restart here will cause us to emit the same dirent to userspace twice

- Fix incorrect checking of the return value on dir_emit(): "true" means
  success, keep going, but bch2_dir_emit() needs to return true when
  we're finished iterating.

https://github.com/koverstreet/bcachefs/issues/867

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