fs-posix: Fix iterating directories when readdir() returns DT_UNKNOWN

Files were iterated correctly, but directories weren't. This mainly broke
directory iteration with NFS when nordirplus mount option was used.

src/lib-fs/fs-posix.c

@@ -844,9 +844,9 @@ static const char *fs_posix_iter_next(struct fs_iter *_iter)
 #ifdef HAVE_DIRENT_D_TYPE
 		switch (d->d_type) {
 		case DT_UNKNOWN:
-			if (!fs_posix_iter_want(iter, d->d_name))
-				break;
-			/* fall through */
+			if (fs_posix_iter_want(iter, d->d_name))
+				return d->d_name;
+			break;
 		case DT_REG:
 		case DT_LNK:
 			if ((iter->iter.flags & FS_ITER_FLAG_DIRS) == 0)