bpf: add napi_id read access to __sk_buff

Add napi_id access to __sk_buff for socket filter program types, tc
program types and other bpf_convert_ctx_access() users. Having access
to skb->napi_id is useful for per RX queue listener siloing, f.e.
in combination with SO_ATTACH_REUSEPORT_EBPF and when busy polling is
used, meaning SO_REUSEPORT enabled listeners can then select the
corresponding socket at SYN time already [1]. The skb is marked via
skb_mark_napi_id() early in the receive path (e.g., napi_gro_receive()).

Currently, sockets can only use SO_INCOMING_NAPI_ID from 6d4339028b
("net: Introduce SO_INCOMING_NAPI_ID") as a socket option to look up
the NAPI ID associated with the queue for steering, which requires a
prior sk_mark_napi_id() after the socket was looked up.

Semantics for the __sk_buff napi_id access are similar, meaning if
skb->napi_id is < MIN_NAPI_ID (e.g. outgoing packets using sender_cpu),
then an invalid napi_id of 0 is returned to the program, otherwise a
valid non-zero napi_id.

  [1] http://netdevconf.org/2.1/slides/apr6/dumazet-BUSY-POLLING-Netdev-2.1.pdf

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/bpf/test_verifier.c

@@ -772,6 +772,9 @@ static struct bpf_test tests[] = {
 			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
 				    offsetof(struct __sk_buff, vlan_tci)),
 			BPF_JMP_IMM(BPF_JGE, BPF_REG_0, 0, 0),
+			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
+				    offsetof(struct __sk_buff, napi_id)),
+			BPF_JMP_IMM(BPF_JGE, BPF_REG_0, 0, 0),
 			BPF_EXIT_INSN(),
 		},
 		.result = ACCEPT,