Slab reclaimable support in Linux

Hello,

On servers that deal with lots of files, the Linux dentry cache takes up a large portion of the total memory. This is reflected in the SReclaimable in /proc/meminfo

MemFree in /proc/meminfo does not subtract this amount, even though 'technically' it is free (where free is defined as usable by user-processes). 

Better visibility into this memory use should reduce freakouts. The attached patch adds this to the core memory module.