btrfs: use helper to find first ref head at btrfs_destroy_delayed_refs()

Instead of open coding it, use the find_first_ref_head() helper at
btrfs_destroy_delayed_refs(). This avoids duplicating the logic,
specially with the upcoming changes in subsequent patches.

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>