NFS: Add mount options for controlling the lookup cache

Add the following NFS-specific mount options to the parser.

    -o lookupcache=all          /* Default: cache positive & negative
                                   dentries */
    -o lookupcache=pos[itive]   /* Don't cache negative dentries */
    -o lookupcache=none         /* Strict revalidation of all dentries */

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>