There is no need to mark this device node to use Giant. The only
architectures that use io(4) (i386 and amd64) only change a flag in
td->td_frame, which is only accessed by curthread.
Apart from this change, I think some fishy things may happen when using
/dev/io in multithreaded applications. I haven't tested, but looking at
the code, the flag doesn't get cleared when close() is called from
another thread, but this may not be this important.
I'm not removing D_NEEDGIANT from mem(4), because this driver isn't
Giant safe at all (it calls GIANT_REQUIRED).
cacdd70cc7