Since commit d2005e3f ("userfaultfd: don't pin the user memory in
userfaultfd_file_create()") userfaultfd uses mm_count rather than
mm_users to pin mm_struct.

Make dup_userfaultfd consistent with this behaviour

Link: http://lkml.kernel.org/r/20161216144821.5183-11-aarcange@redhat.comSigned-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Michael Rapoport <RAPOPORT@il.ibm.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>