mm/memcontrol: make mem_cgroup_inactive_anon_is_low() return bool

Make mem_cgroup_inactive_anon_is_low return bool due to this particular
function only using either one or zero as its return value.

No functional change.

Signed-off-by: Yaowei Bai <bywxiaobai@163.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>