btrfs: assert tree lock is held when linking free space

When linking a free space entry, at link_free_space(), the caller should
be holding the spinlock 'tree_lock' of the given btrfs_free_space_ctl
argument, which is necessary for manipulating the red black tree of free
space entries (done by tree_insert_offset(), which already asserts the
lock is held) and for manipulating the 'free_space', 'free_extents',
'discardable_extents' and 'discardable_bytes' counters of the given
struct btrfs_free_space_ctl.

So assert that the spinlock 'tree_lock' of the given btrfs_free_space_ctl
is held by the current task. We have multiple code paths that end up
calling link_free_space(), and all currently take the lock before calling
it.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/free-space-cache.c

@@ -1850,6 +1850,8 @@ static int link_free_space(struct btrfs_free_space_ctl *ctl,
 {
 	int ret = 0;
 
+	lockdep_assert_held(&ctl->tree_lock);
+
 	ASSERT(info->bytes || info->bitmap);
 	ret = tree_insert_offset(ctl, NULL, info);
 	if (ret)