bpf: Add bpf_skc_to_udp6_sock() helper

The helper is used in tracing programs to cast a socket
pointer to a udp6_sock pointer.
The return value could be NULL if the casting is illegal.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Cc: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/bpf/20200623230815.3988481-1-yhs@fb.com