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

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

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

Signed-off-by: Kyeong Yoo <kyeong.yoo@alliedtelesis.co.nz>