SUNRPC: Use soft connect semantics when performing RPC ping

Currently, if a remote RPC service is unreachable, an RPC ping will
hang until the underlying transport connect attempt times out.  A more
desirable behavior might be to have the ping fail immediately so upper
layers can recover appropriately.

In the case of an NFS mount, for instance, this would mean the
mount(2) system call could fail immediately if the server isn't
listening, rather than hanging uninterruptibly for more than 3
minutes.

Change rpc_ping() so that it fails immediately for connection-oriented
transports.  rpc_create() will then fail immediately for such
transports if an RPC ping was requested.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>