NFS: mount option parser chokes on proto=

The new text-based NFS mount option parsing logic doesn't recognize any
valid transport protocols due to a silly mistake in the protocol token
matching logic.  This prevents basic mount requests such as:

   mount.nfs server:/export /mnt -o proto=tcp

from working with the new text-based NFS mount API.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>