This patch fixes a bug on PPC where the kernel will oops if a process
does a system call and the system call number is out of range.

While fixing that, I noticed that if the process is being ptraced, an
out-of-range system call will not get traced on the way in but will on
the way out.  This patch fixes that too, by making it get traced on
the way in as well as the way out.  It turned out to be less change,
and fewer instructions overall, to do that than to make the
out-of-range system call not be traced at all.