UBUNTU: SAUCE: (no-up) add tracing for user initiated readahead requests

UBUNTU: SAUCE: (no-up) add tracing for user initiated readahead requests

Track pages which undergo readahead and for each record which were
actually consumed, via either read or faulted into a map.  This allows
userspace readahead applications (such as ureadahead) to track which
pages in core at the end of a boot are actually required and generate an
optimal readahead pack.  It also allows pack adjustment and optimisation
in parallel with readahead, allowing the pack to evolve to be accurate
as userspace paths change.  The status of the pages are reported back via
the mincore() call using a newly allocated bit.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Leann Ogasawara <leann.ogasawara@canonical.com>