NFS: Add FS-Cache option bit and debug bit

Add FS-Cache option bit to nfs_server struct.  This is set to indicate local
on-disk caching is enabled for a particular superblock.

Also add debug bit for local caching operations.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>