NFS: split cache creation and PipeFS registration

This precursor patch splits NFS cache creation and PipeFS registartion.
It's required for latter split of NFS DNS resolver cache creation per network
namespace context and PipeFS registration/unregistration on MOUNT/UMOUNT
events.

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