mm, vmscan: add mm_vmscan_inactive_list_is_low tracepoint

mm, vmscan: add mm_vmscan_inactive_list_is_low tracepoint

Currently we have tracepoints for both active and inactive LRU lists
reclaim but we do not have any which would tell us why we we decided to
age the active list.  Without that it is quite hard to diagnose
active/inactive lists balancing.  Add mm_vmscan_inactive_list_is_low
tracepoint to tell us this information.

Link: http://lkml.kernel.org/r/20170104101942.4860-8-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>