Btrfs: Add readahead to the online shrinker, and a mount -o alloc_start= for testing

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index b83a1be..d6e5c19 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -324,6 +324,7 @@
 	u64 last_trans_committed;
 	unsigned long mount_opt;
 	u64 max_extent;
+	u64 alloc_start;
 	struct btrfs_transaction *running_transaction;
 	struct btrfs_super_block super_copy;
 	struct extent_buffer *sb_buffer;