NFS: Fix a few constant_table array definitions

nfs_vers_tokens, nfs_xprt_protocol_tokens, and nfs_secflavor_tokens were
all missing an empty item at the end of the array, allowing
lookup_constant() to potentially walk off the end and trigger and oops.

Reported-by: Olga Kornievskaia <aglo@umich.edu>
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Fixes: e38bb238 ("NFS: Convert mount option parsing to use functionality from fs_parser.h")
Cc: stable@vger.kernel.org # v5.6
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>