page-read: Introduce PR_ASAP flag for read_pages

This flag means, that the PR_ASYNC is valid, but the IO
should be started ASAP. This is how remote reader works,
so this flag is mostly for the local reader. It will let
us unify page-fault handlers for local and remote cases.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>