net/9p: distinguish zero-copy requests

Add boolean `zc` member to struct p9_fcall to distinguish zero-copy
messages (not using the linear `sdata` buffer for message payload) from
regular messages (which do copy message payload to `sdata` before being
further processed).

This new member is appended to end of structure to avoid inserting huge
padding in generated layout.

Link: https://lkml.kernel.org/r/8f2a5c12a446c3b544da64e0b1550e1fb2d6f972.1669144861.git.linux_oss@crudebyte.com
Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
diff --git a/net/9p/client.c b/net/9p/client.c
index b554f83..a2b4a96 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -685,6 +685,9 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
 	if (IS_ERR(req))
 		return req;
 
+	req->tc.zc = false;
+	req->rc.zc = false;
+
 	if (signal_pending(current)) {
 		sigpending = 1;
 		clear_thread_flag(TIF_SIGPENDING);
@@ -783,6 +786,9 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
 	if (IS_ERR(req))
 		return req;
 
+	req->tc.zc = true;
+	req->rc.zc = true;
+
 	if (signal_pending(current)) {
 		sigpending = 1;
 		clear_thread_flag(TIF_SIGPENDING);