Use enumeration to indicate usage level of swap space and simplify code.
Both high and low watermark are too low at today's hardware, like 1 ~ 10 GB
while it was large enough numbers 20 years ago 16 ~ 64 MB.
I'd like to propose to change 1/16 as high watermark and 1/32 as low one
to adjust at swapon time (as a separate and next changeset.)