Staging: batman-adv: return -EFAULT on copy_to_user errors

copy_to_user() returns the number of bites remaining but we want to
return a negative error code here.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>