SUNRPC: Lock the transport layer on shutdown

Avoid all races with the connect/disconnect handlers by taking the
transport lock.

Reported-by: "Suzuki K. Poulose" <suzuki.poulose@arm.com>
Acked-by: Jeff Layton <jlayton@poochiereds.net>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>