Documentation: add watermark_scale_factor to the list of vm systcl file

Documentation: add watermark_scale_factor to the list of vm systcl file

Commit 795ae7a0de6b ("mm: scale kswapd watermarks in proportion to
memory") properly added the description of the new knob to
Documentation/sysctl/vm.txt, but forgot to add it to the list of files
in /proc/sys/vm. Let's fix that.

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
(cherry picked from commit e6507a00fd08986ce003012a10af78cc7e47eee8)

Orabug: 26643957

Signed-off-by: Robert M. Harris <robert.m.harris@oracle.com>
Reviewed-by: Khalid Aziz <khalid.aziz@oracle.com>
Reviewed-by: Larry Bassel <larry.bassel@oracle.com>
Reviewed-by: Herbert van den Bergh <herbert.van.den.bergh@oracle.com>
Reviewed-by: Todd Vierling <todd.vierling@oracle.com>