btrfs: assert delayed refs lock is held at find_ref_head()

We have 3 callers for find_ref_head() so assert at find_ref_head() that we
have the delayed refs lock held, removing the assertion from one of its
callers (btrfs_find_delayed_ref_head()).

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.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/delayed-ref.c

@@ -407,6 +407,8 @@ static struct btrfs_delayed_ref_head *find_ref_head(
 	struct rb_node *n;
 	struct btrfs_delayed_ref_head *entry;
 
+	lockdep_assert_held(&dr->lock);
+
 	n = root->rb_node;
 	entry = NULL;
 	while (n) {
@@ -1195,8 +1197,6 @@ btrfs_find_delayed_ref_head(const struct btrfs_fs_info *fs_info,
 			    struct btrfs_delayed_ref_root *delayed_refs,
 			    u64 bytenr)
 {
-	lockdep_assert_held(&delayed_refs->lock);
-
 	return find_ref_head(fs_info, delayed_refs, bytenr, false);
 }