nfs: Add "lookupcache" to displayed mount options

Running "cat /proc/mounts" fails to display the "lookupcache" option.
This oversight cost me a bunch of wasted time recently.

The following simple patch fixes it.

CC: stable <stable@kernel.org>
Signed-off-by: Patrick LoPresti <lopresti@gmail.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>