plist: Add plist_last

plist is currently used by the scheduler, which only needs to know the
highest item in the list.  This adds plist_last which allows you to
find the lowest.  This is necessary for using plists to implement a
fast search of dynamic ranges in pm_qos which can have both highest
and lowest criteria.

Signed-off-by: James Bottomley <James.Bottomley@suse.de>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>