Remove unneeded D_NEEDGIANT from /dev/fd/{0,1,2}.

There is no reason the fdopen() routine needs Giant. It only sets
curthread->td_dupfd, based on the device unit number of the cdev.

I guess we won't get massive performance improvements here, but still, I
assume we eventually want to get rid of Giant.

sys/kern/kern_descrip.c

@@ -2972,7 +2972,6 @@ fdopen(struct cdev *dev, int mode, int type, struct thread *td)
 
 static struct cdevsw fildesc_cdevsw = {
 	.d_version =	D_VERSION,
-	.d_flags =	D_NEEDGIANT,
 	.d_open =	fdopen,
 	.d_name =	"FD",
 };