mavlink_ftp: avoid using seekdir()

The provided argument payload->offset is in range [0, num_file_entries-1],
but seekdir might use a completely different range. It is not defined
by the API spec. It is only useful in conjunction with telldir().

src/modules/mavlink/mavlink_ftp.cpp

@@ -311,7 +311,8 @@ MavlinkFTP::_workList(PayloadHeader *payload, bool list_hidden)
 	struct dirent *result = nullptr;
 
 	// move to the requested offset
-	seekdir(dp, payload->offset);
+	int requested_offset = payload->offset;
+	while (requested_offset-- > 0 && readdir(dp));
 
 	for (;;) {
 		errno = 0;