This patch fixes a fairly serious tlb flushing bug that makes aio use under
uml very unreliable -- SEGVs, Oops and panic()s occur as a result of stale
tlb entires being used by uml when aio switches mms due to the fact that
uml does not implement the activate_mm() hook.  This patch introduces a
simple but correct approach (read: hammer) for implementing activate_mm()
in uml by doing a force_flush_all() if the new mm is different from old.
With this patch in place, uml is able to succeed at the aio test case that
was randomly faulting for me before.

Cc: Jeff Dike <jdike@addtoit.com>
Cc: <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>