linux-user: don't swap NLMSG_DATA() fields

If the structure pointed by NLMSG_DATA() is bigger
than the size of NLMSG_DATA(), don't swap its fields
to avoid memory corruption.

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>