mm: allow __get_user_pages() callers to avoid triggering page faults.

mm: allow __get_user_pages() callers to avoid triggering page faults.

Some callers of __get_userpages() would like to tentatively probe for
pages which may or may not be faulted in, in contexts in which faulting
is prohibited.  __get_user_pages() can return early before the fault is
complete, but cannot avoid the fault entirely.

This change introduces FOLL_NOFAULT, which causes an early return if a
fault would occur, without triggering a fault, and indicating (if the
caller requests it) that the call would block.

Orabug: 18412802
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>
Reviewed-by: Chuck Anderson <chuck.anderson@oracle.com>