linux-user: fd_trans_host_to_target_data() must process only received data

if we process the whole buffer, the netlink helpers can try
to swap invalid data.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>