bpf: Add ipip6 and ip6ip decap support for bpf_skb_adjust_room()

Add ipip6 and ip6ip decap support for bpf_skb_adjust_room().
Main use case is for using cls_bpf on ingress hook to decapsulate
IPv4 over IPv6 and IPv6 over IPv4 tunnel packets.

Add two new flags BPF_F_ADJ_ROOM_DECAP_L3_IPV{4,6} to indicate the
new IP header version after decapsulating the outer IP header.

Suggested-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/b268ec7f0ff9431f4f43b1b40ab856ebb28cb4e1.1673574419.git.william.xuanziyang@huawei.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

include/uapi/linux/bpf.h

@@ -2647,6 +2647,11 @@ union bpf_attr {
  *		  Use with BPF_F_ADJ_ROOM_ENCAP_L2 flag to further specify the
  *		  L2 type as Ethernet.
  *
+ *		* **BPF_F_ADJ_ROOM_DECAP_L3_IPV4**,
+ *		  **BPF_F_ADJ_ROOM_DECAP_L3_IPV6**:
+ *		  Indicate the new IP header version after decapsulating the outer
+ *		  IP header. Used when the inner and outer IP versions are different.
+ *
  * 		A call to this helper is susceptible to change the underlying
  * 		packet buffer. Therefore, at load time, all checks on pointers
  * 		previously done by the verifier are invalidated and must be
@@ -5807,6 +5812,8 @@ enum {
 	BPF_F_ADJ_ROOM_ENCAP_L4_UDP	= (1ULL << 4),
 	BPF_F_ADJ_ROOM_NO_CSUM_RESET	= (1ULL << 5),
 	BPF_F_ADJ_ROOM_ENCAP_L2_ETH	= (1ULL << 6),
+	BPF_F_ADJ_ROOM_DECAP_L3_IPV4    = (1ULL << 7),
+	BPF_F_ADJ_ROOM_DECAP_L3_IPV6    = (1ULL << 8),
 };
 
 enum {