When mremap is called with MREMAP_FIXED it unmaps memory at the
destination address without notifying userfaultfd monitor.

If the destination were registered with userfaultfd, the monitor has no
way to distinguish between the old and new ranges and to properly relate
the page faults that would occur in the destination region.

Fixes: 897ab3e0 ("userfaultfd: non-cooperative: add event for memory unmaps")
Link: http://lkml.kernel.org/r/1500276876-3350-1-git-send-email-rppt@linux.vnet.ibm.comSigned-off-by: NMike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: NPavel Emelyanov <xemul@virtuozzo.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>