NFS: Set security flavor default for NFSv2/3 mounts like other defaults

Set the default security flavor when we set the other mount option default
values.  After this change, only the legacy user-space mount path needs to
set the NFS_MOUNT_SECFLAVOUR flag.

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