SUNRPC: Make xs_tcp_close() do a socket shutdown rather than a sock_release

SUNRPC: Make xs_tcp_close() do a socket shutdown rather than a sock_release

Use of socket shutdown() means that we monitor the shutdown process
through the xs_tcp_state_change() callback, so it is preferable to
a full close in all cases unless we're destroying the transport.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>