NFS4: Set security flavor default for NFSv4 mounts like other defaults

Set the default security flavor when we set the other mount option
default values for NFSv4.  This cleans up the NFSv4 mount option parsing
path to look like the NFSv2/v3 one.

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