Make the size check of ioctl_send_request and
ioctl_send_broadcast_request speed dependent.  Also change the error
return code from -EINVAL to -EIO to distinguish this from other errors
concerning the ioctl parameters.

Another payload size limit for which we don't check here though is the
remote node's Bus_Info_Block.max_rec.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>