Fix: update vmscan instrumentation for v4.16

Fix: update vmscan instrumentation for v4.16

See upstream commit :

  commit 9092c71bb724dba2ecba849eae69e5c9d39bd3d2
  Author: Josef Bacik <jbacik@fb.com>
  Date:   Wed Jan 31 16:16:26 2018 -0800

    mm: use sc->priority for slab shrink targets

    Previously we were using the ratio of the number of lru pages scanned to
    the number of eligible lru pages to determine the number of slab objects
    to scan.  The problem with this is that these two things have nothing to
    do with each other, so in slab heavy work loads where there is little to
    no page cache we can end up with the pages scanned being a very low
    number.  This means that we reclaim next to no slab pages and waste a
    lot of time reclaiming small amounts of space.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>