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>

fs/bcachefs/dirent.c

@@ -685,7 +685,7 @@ static int bch2_dir_emit(struct dir_context *ctx, struct bkey_s_c_dirent d, subv
 		      vfs_d_type(d.v->d_type));
 	if (ret)
 		ctx->pos = d.k->p.offset + 1;
-	return ret;
+	return !ret;
 }
 
 int bch2_readdir(struct bch_fs *c, subvol_inum inum, struct dir_context *ctx)
@@ -710,7 +710,7 @@ int bch2_readdir(struct bch_fs *c, subvol_inum inum, struct dir_context *ctx)
 			if (ret2 > 0)
 				continue;
 
-			ret2 ?: drop_locks_do(trans, bch2_dir_emit(ctx, dirent, target));
+			ret2 ?: (bch2_trans_unlock(trans), bch2_dir_emit(ctx, dirent, target));
 		})));
 
 	bch2_bkey_buf_exit(&sk, c);