btrfs: send: allow protocol version 3 with CONFIG_BTRFS_DEBUG

We haven't finalized send stream v3 yet, so gate the send stream version
behind CONFIG_BTRFS_DEBUG as we want some way to test it.

The original verity send did not check the protocol version, so add that
actual protection as well.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/send.c

@@ -6469,7 +6469,7 @@ static int finish_inode_if_needed(struct send_ctx *sctx, int at_end)
 		if (ret < 0)
 			goto out;
 	}
-	if (sctx->cur_inode_needs_verity) {
+	if (sctx->proto >= 3 && sctx->cur_inode_needs_verity) {
 		ret = process_verity(sctx);
 		if (ret < 0)
 			goto out;

fs/btrfs/send.h

@@ -10,7 +10,12 @@
 #include <linux/types.h>
 
 #define BTRFS_SEND_STREAM_MAGIC "btrfs-stream"
+/* Conditional support for the upcoming protocol version. */
+#ifdef CONFIG_BTRFS_DEBUG
+#define BTRFS_SEND_STREAM_VERSION 3
+#else
 #define BTRFS_SEND_STREAM_VERSION 2
+#endif
 
 /*
  * In send stream v1, no command is larger than 64K. In send stream v2, no limit