NFS: handle NFS caches dentries by network namespace aware routines

This patch makes NFS caches PipeFS dentries allocated and destroyed in network
namespace context by PipeFS network namespace aware routines.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>