Doing piggyback on struct ifreq for passing arguments to CAN-specific
control requests is unsafe since nothing relates sizeof(ifr.ifr_ifru)
to the largest data field we need. Besides, this is prone to type
aliasing issues when mapping the CAN members over the union space
using casts, which the compiler may complain about loudly.

Introduce struct can_ifreq, defining all the data fields currently
supported by CAN raw ioctl requests. The driver still accepts struct
ifreq from existing applications though, but it is recommended to
switch to can_ifreq at the first opportunity.