Don't go via map->ops to call sock_map_update_elem, since we know
what function to call in bpf_map_update_value. Since
check_map_func_compatibility doesn't allow calling
BPF_FUNC_map_update_elem from BPF context, we can remove
ops->map_update_elem and rename the function to
sock_map_update_elem_sys.
Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>