nfs: use vfs cache

make use of attribute caching. Three new options are added:

    nfs.namespace-cache.time
    nfs.namespace-cache.unit
    nfs.namespace-cache.size

The default value for caching time is 3 sec, which an optimal for most workloads.
The default cache size (number of cached objects) is zero, which is effectively, switching off
the caching.

Each caches entry takes 120 bytes of additional memory.

Acked-by: Paul Millar
Target: master
Require-book: no
Require-notes: yes