NFS: Attempt mount with default sec flavor first

nfs4_lookup_root() is already configured to use either RPC_AUTH_UNIX
or a user specified flavor (through -o sec=<whatever>).  We should
use this flavor first, and only attempt negotiation if it fails
with -EPERM.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>