rpc client can not deal with ENOSOCK, so translate it into ENOCONN

If NFSv4 client send a request before connect, or the old connection was broken
because a ETIMEOUT error catched by call_status, ->send_request will return
ENOSOCK, but rpc layer can not deal with it, so make sure ->send_request can
translate ENOSOCK into ENOCONN.

Signed-off-by: Bian Naimeng <biannm@cn.fujitsu.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>