SUNRPC: Fix RPCAUTH_LOOKUP_ROOTCREDS

The current RPCAUTH_LOOKUP_ROOTCREDS flag only works for AUTH_SYS
authentication, and then only as a special case in the code. This patch
removes the auth_sys special casing, and replaces it with generic code.

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