SUNRPC: Use TCP_CORK to optimise send performance on the server

Use a counter to keep track of how many requests are queued behind the
xprt->xpt_mutex, and keep TCP_CORK set until the queue is empty.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Link: https://lore.kernel.org/linux-nfs/20210213202532.23146-1-trondmy@kernel.org/T/#u
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/svcsock.h

@@ -35,6 +35,8 @@ struct svc_sock {
 	/* Total length of the data (not including fragment headers)
 	 * received so far in the fragments making up this rpc: */
 	u32			sk_datalen;
+	/* Number of queued send requests */
+	atomic_t		sk_sendqlen;
 
 	struct page *		sk_pages[RPCSVC_MAXPAGES];	/* received data */
 };