NFS: Use the authentication flavor list returned by mountd

Commit a14017db

 added support in the kernel's NFS mount client to
decode the authentication flavor list returned by mountd.

The NFS client can now use this list to determine whether the
authentication flavor requested by the user is actually supported
by the server.

Note we don't actually negotiate the security flavor if none was
specified by the user.  Instead, we try to use AUTH_SYS, and fail if
the server does not support it.  This prevents us from negotiating
an inappropriate security flavor (some servers list AUTH_NULL first).

If the server does not support AUTH_SYS, the user must provide an
appropriate security flavor by specifying the "sec=" mount option.

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