SUNRPC: Fix a potential memory leak in rpc_new_client

If the call to rpciod_up() fails, we currently leak a reference to the
struct rpc_xprt.
As part of the fix, we also remove the redundant check for xprt!=NULL.
This is already taken care of by the callers.

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