btrfs-progs: enable send v3 correctly (use EXPERIMENTAL instead of CONFIG_BTRFS_DEBUG)

The send v3 protocol is enabled in kernel by a different config option
than in btrfs-progs to actually work. Now v3 can be tested when
configured and built with --enable-experimental.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: boryas

kernel-shared/send.h

@@ -25,7 +25,11 @@
 
 #define BTRFS_SEND_STREAM_MAGIC "btrfs-stream"
 /* Conditional support for the upcoming protocol version. */
-#ifdef CONFIG_BTRFS_DEBUG
+/*
+ * Note: this is CONFIG_BTRFS_DEBUG in kernel but for btrfs-progs it must be
+ * enabled in experimental mode.
+ */
+#if EXPERIMENTAL
 #define BTRFS_SEND_STREAM_VERSION 3
 #else
 #define BTRFS_SEND_STREAM_VERSION 2