NFS: Handle a zero-length auth flavor list

Some releases of Linux rpc.mountd (nfs-utils 1.1.4 and later) return an
empty auth flavor list if no sec= was specified for the export.  This is
notably broken server behavior.

The new auth flavor list checking added in a recent commit rejects this
case.  The OpenSolaris client does too.

The broken mountd implementation is already widely deployed.  To avoid
a behavioral regression, the kernel's mount client skips flavor checking
(ie reverts to the pre-2.6.32 behavior) if mountd returns an empty
flavor list.

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