sunrpc: Adjust size of socket's receive page array dynamically

As a step towards making NFSD's maximum rsize and wsize variable at
run-time, make sk_pages a flexible array.

Reviewed-by: Jeff Layton <[e-mail]>
Reviewed-by: NeilBrown <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

include/linux/sunrpc/svcsock.h

···
 
 	struct completion	sk_handshake_done;
 
-	struct page *		sk_pages[RPCSVC_MAXPAGES];	/* received data */
+	/* received data */
+	unsigned long		sk_maxpages;
+	struct page *		sk_pages[] __counted_by(sk_maxpages);
 };
 
 static inline u32 svc_sock_reclen(struct svc_sock *svsk)

net/sunrpc/svcsock.c

···
 		svsk->sk_marker = xdr_zero;
 		svsk->sk_tcplen = 0;
 		svsk->sk_datalen = 0;
-		memset(&svsk->sk_pages[0], 0, sizeof(svsk->sk_pages));
+		memset(&svsk->sk_pages[0], 0,
+		       svsk->sk_maxpages * sizeof(struct page *));
 
 		tcp_sock_set_nodelay(sk);
 
···
 	struct svc_sock	*svsk;
 	struct sock	*inet;
 	int		pmap_register = !(flags & SVC_SOCK_ANONYMOUS);
+	unsigned long	pages;
 
-	svsk = kzalloc(sizeof(*svsk), GFP_KERNEL);
+	pages = svc_serv_maxpages(serv);
+	svsk = kzalloc(struct_size(svsk, sk_pages, pages), GFP_KERNEL);
 	if (!svsk)
 		return ERR_PTR(-ENOMEM);
+	svsk->sk_maxpages = pages;
 
 	inet = sock->sk;