sunrpc: Add a helper to derive maxpages from sv_max_mesg

This page count is to be used to allocate various arrays of pages
and bio_vecs, replacing the fixed RPCSVC_MAXPAGES value.

The documenting comment is somewhat stale -- of course NFSv4
COMPOUND procedures may have multiple payloads.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/svc.h

@@ -159,6 +159,23 @@ extern u32 svc_max_payload(const struct svc_rqst *rqstp);
 #define RPCSVC_MAXPAGES		((RPCSVC_MAXPAYLOAD+PAGE_SIZE-1)/PAGE_SIZE \
 				+ 2 + 1)
 
+/**
+ * svc_serv_maxpages - maximum count of pages needed for one RPC message
+ * @serv: RPC service context
+ *
+ * Returns a count of pages or vectors that can hold the maximum
+ * size RPC message for @serv.
+ *
+ * Each request/reply pair can have at most one "payload", plus two
+ * pages, one for the request, and one for the reply.
+ * nfsd_splice_actor() might need an extra page when a READ payload
+ * is not page-aligned.
+ */
+static inline unsigned long svc_serv_maxpages(const struct svc_serv *serv)
+{
+	return DIV_ROUND_UP(serv->sv_max_mesg, PAGE_SIZE) + 2 + 1;
+}
+
 /*
  * The context of a single thread, including the request currently being
  * processed.