sched/x86: Add a comment clarifying LDT context switching

The code is correct, but only for a rather subtle reason.  This
confused me for quite a while when I read switch_mm, so clarify
the code to avoid confusing other people, too.

TBH, I wouldn't be surprised if this code was only correct by
accident.

[ I wouldn't normally send a comment-only patch, but it took me a long
  time to first figure out wtf was going on here, and then to figure
  out why this wasn't exploitable by malicious code, and then to
  figure out why this oddity had no user-visible effect at all.  Let's
  spare future readers the same confusion. ]

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/36275c99801a87d8dcf0502a41cf4e2ad81aae46.1412623954.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>