Properly check the return value of copy_from/to_user() and handle any
errors that might happen.

This removes a bunch of compiler warnings.

Cc: Alois Schlögl <alois.schloegl@ist.ac.at>
Cc: Greg P. Smith <greg@ced.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>