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/inode.c b/fs/btrfs/inode.c
index 3c7bf3b..d6de469 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -2058,7 +2058,7 @@
 
 	mutex_lock(&root->fs_info->fs_mutex);
 	ret = btrfs_check_free_space(root, PAGE_CACHE_SIZE, 0);
-	mutex_lock(&root->fs_info->fs_mutex);
+	mutex_unlock(&root->fs_info->fs_mutex);
 	if (ret)
 		goto out;