btrfs: assert tree lock is held when searching for free space entries

When searching for a free space entry by offset, at tree_search_offset(),
we are supposed to have the btrfs_free_space_ctl's 'tree_lock' held, so
assert that. We have multiple callers of tree_search_offset(), and all
currently hold the necessary lock before calling it.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/free-space-cache.c

@@ -1721,6 +1721,8 @@ tree_search_offset(struct btrfs_free_space_ctl *ctl,
 	struct rb_node *n = ctl->free_space_offset.rb_node;
 	struct btrfs_free_space *entry = NULL, *prev = NULL;
 
+	lockdep_assert_held(&ctl->tree_lock);
+
 	/* find entry that is closest to the 'offset' */
 	while (n) {
 		entry = rb_entry(n, struct btrfs_free_space, offset_index);