mm: mmu_notifier: fix inconsistent memory between secondary MMU and host

There is a bug in set_pte_at_notify() which always sets the pte to the
new page before releasing the old page in the secondary MMU.  At this
time, the process will access on the new page, but the secondary MMU
still access on the old page, the memory is inconsistent between them

The below scenario shows the bug more clearly:

at the beginning: *p = 0, and p is write-protected by KSM or shared with
parent process

CPU 0                                       CPU 1
write 1 to p to trigger COW,
set_pte_at_notify will be called:
  *pte = new_page + W; /* The W bit of pte is set */

                                     *p = 1; /* pte is valid, so no #PF */

                                     return back to secondary MMU, then
                                     the secondary MMU read p, but get:
                                     *p == 0;

                         /*
                          * !!!!!!
                          * the host has already set p to 1, but the secondary
                          * MMU still get the old value 0
                          */

  call mmu_notifier_change_pte to release
  old page in secondary MMU

We can fix it by release old page first, then set the pte to the new
page.

Note, the new page will be firstly used in secondary MMU before it is
mapped into the page table of the process, but this is safe because it
is protected by the page table lock, there is no race to change the pte

[akpm@linux-foundation.org: add comment from Andrea]
Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>