linux: Make the macro LINUX_IOCTL_SET public

There're some other drivers want to register and unregister linux
ioctl handler. Move the macro LINUX_IOCTL_SET from tdfx_linux.h to
linux_ioctl.h so that they can also benefit it.

While here, rename the declaration of linux ioctl function to be
consistent with the name of the handler.

Meanwhile, drop a comment about the macro, since its function is
obvious.

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D53158