btrfs: send add define for v2 buffer size

Add a define for the data buffer size (though the maximum size is not
limited by it) BTRFS_SEND_BUF_SIZE_V2 so it's more visible.
Signed-off-by: Wang Yugui <wangyugui@e16-tech.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/send.c

@@ -7901,7 +7901,7 @@ long btrfs_ioctl_send(struct inode *inode, struct btrfs_ioctl_send_args *arg)
 	if (sctx->proto >= 2) {
 		u32 send_buf_num_pages;
 
-		sctx->send_max_size = ALIGN(SZ_16K + BTRFS_MAX_COMPRESSED, PAGE_SIZE);
+		sctx->send_max_size = BTRFS_SEND_BUF_SIZE_V2;
 		sctx->send_buf = vmalloc(sctx->send_max_size);
 		if (!sctx->send_buf) {
 			ret = -ENOMEM;

fs/btrfs/send.h

@@ -18,10 +18,12 @@
 #endif
 
 /*
- * In send stream v1, no command is larger than 64K. In send stream v2, no limit
- * should be assumed.
+ * In send stream v1, no command is larger than 64K. In send stream v2, no
+ * limit should be assumed, the buffer size is set to be a header with
+ * compressed extent size.
  */
 #define BTRFS_SEND_BUF_SIZE_V1				SZ_64K
+#define BTRFS_SEND_BUF_SIZE_V2	ALIGN(SZ_16K + BTRFS_MAX_COMPRESSED, PAGE_SIZE)
 
 struct inode;
 struct btrfs_ioctl_send_args;