Changes since v1:
1) Fix bug in kill_me_never() that forgot to clear p->mce_count so
  repeated recovery in the same task would trigger the panic for
"Machine checks to different user pages"
  [Note to Jue Wang ... this *might* be why your test that injects
   two errors into the same buffer passed to a write(2) syscall
   failed with this message]

I recreated Jue's specific test today with uncorrected errors in two
pages passed to a write(2) syscall.

	buf = alloc(2 pages);
	inject(buf + 0x440);
	inject*buf + 0x11c0);
	n = write(fd, buf, 8K);

Result was that the write returned 0x440 (i.e. bytes written up to the
first poison location).

-Tony