gup: Introduce FOLL_NOFAULT flag to disable page faults

Introduce a new FOLL_NOFAULT flag that causes get_user_pages to return
-EFAULT when it would otherwise trigger a page fault.  This is roughly
similar to FOLL_FAST_ONLY but available on all architectures, and less
fragile.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>