proc: fix MemAvailable in /proc/meminfo to exclude tmpfs files

[kyeongy]

The "total_cache" from memory.stat of cgroup includes the memory used by tmpfs files ("total_shmem"). Considering it as available memory is wrong because files created on a tmpfs file system cannot be simply reclaimed.

So the available memory is calculated with the sum of:

• Memory the kernel knows is free
• Memory that contained in the kernel active file LRU, that can be reclaimed if necessary
• Memory that is contained in the kernel non-active file LRU, that can be reclaimed if necessary

[mihalicyn]

Considering it as available memory is wrong because files created on a tmpfs file system cannot be simply reclaimed.

It can be reclaimed in fact (to the swap).

But anyways, this change looks good to me.

Thanks!