proc_vmstat.c: update commented list of metrics from /proc/vmstat, NFC

proc_vmstat.c: update commented list of metrics from /proc/vmstat, NFC

These are the metrics available on one of my systems running Linux 4.9.2
with this[1] kernel config.

This commit should not introduce any functional changes, and is only
a preparatory step for adding NUMA, NUMA balancing, and other stats.

[1] https://git.uplinklabs.net/steven/projects/archlinux/ec2/ec2-packages.git/tree/linux-ec2/config.x86_64?id=7a8cae7f38faf6f422095c53f2762092e788a44c

Signed-off-by: Steven Noonan <steven@uplinklabs.net>