mm: fix __gup_device_huge vs unmap

mm: fix __gup_device_huge vs unmap

BugLink: http://bugs.launchpad.net/bugs/1807469
commit a9b6de77b1a3ff729f7bfc54b2e17711776a416c upstream.

get_user_pages_fast() for device pages is missing the typical validation
that all page references have been taken while the mapping was valid.
Without this validation truncate operations can not reliably coordinate
against new page reference events like O_DIRECT.

Cc: <stable@vger.kernel.org>
Fixes: 3565fce3a659 ("mm, x86: get_user_pages() for dax mappings")
Reported-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>