Revert "mm: correctly synchronize rss-counters at exit/exec"

This reverts commit 40af1bbdca.

It's horribly and utterly broken for at least the following reasons:

 - calling sync_mm_rss() from mmput() is fundamentally wrong, because
   there's absolutely no reason to believe that the task that does the
   mmput() always does it on its own VM.  Example: fork, ptrace, /proc -
   you name it.

 - calling it *after* having done mmdrop() on it is doubly insane, since
   the mm struct may well be gone now.

 - testing mm against NULL before you call it is insane too, since a
NULL mm there would have caused oopses long before.

.. and those are just the three bugs I found before I decided to give up
looking for me and revert it asap.  I should have caught it before I
even took it, but I trusted Andrew too much.

Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Markus Trippelsdorf <markus@trippelsdorf.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

@@ -619,14 +619,6 @@ void mmput(struct mm_struct *mm)
 			module_put(mm->binfmt->module);
 		mmdrop(mm);
 	}
-
-	/*
-	 * Final rss-counter synchronization. After this point there must be
-	 * no pagefaults into this mm from the current context.  Otherwise
-	 * mm->rss_stat will be inconsistent.
-	 */
-	if (mm)
-		sync_mm_rss(mm);
 }
 EXPORT_SYMBOL_GPL(mmput);