NFS: Mount option parser should detect missing "port="

The meaning of not specifying the "port=" mount option is different
for "-t nfs" and "-t nfs4" mounts.  The default port value for
NFSv2/v3 mounts is 0, but the default for NFSv4 mounts is 2049.

To support "-t nfs -o vers=4", the mount option parser must detect
when "port=" is missing so that the correct default port value can be
set depending on which NFS version is requested.

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