mm: vmscan: move inactive_list_is_low() swap check to the caller

inactive_list_is_low() should be about one thing: checking the ratio
between inactive and active list.  Kitchensink checks like the one for
swap space makes the function hard to use and modify its callsites.
Luckly, most callers already have an understanding of the swap situation,
so it's easy to clean up.

get_scan_count() has its own, memcg-aware swap check, and doesn't even get
to the inactive_list_is_low() check on the anon list when there is no swap
space available.

shrink_list() is called on the results of get_scan_count(), so that check
is redundant too.

age_active_anon() has its own totalswap_pages check right before it checks
the list proportions.

The shrink_node_memcg() site is the only one that doesn't do its own swap
check.  Add it there.

Then delete the swap check from inactive_list_is_low().

Link: http://lkml.kernel.org/r/20191022144803.302233-4-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>