SUNRPC: Get rid of the redundant xprt->shutdown bit field

It is only set after everyone has dereferenced the transport,
and serves no useful purpose: setting it is racy, so all the
socket code, etc still needs to be able to cope with the cases
where they miss reading it.

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