WIP: fix sock_addr_is_valid_access() for 32-bit

Support 32-bit pointers and fix errors like:

root@qemu-armhf:/usr/libexec/kselftests-bpf# ./test_progs -w 0 -a socket_cookie
libbpf: prog 'set_cookie': BPF program load failed: -EACCES
libbpf: prog 'set_cookie': -- BEGIN PROG LOAD LOG --
0: R1=ctx() R10=fp0
; if (ctx->family != AF_INET6 || ctx->user_family != AF_INET6) @ socket_cookie_prog.c:37
0: (61) r2 = *(u32 *)(r1 +28)         ; R1=ctx() R2_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
1: (56) if w2 != 0xa goto pc+16       ; R2_w=10
2: (61) r2 = *(u32 *)(r1 +0)          ; R1=ctx() R2_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
3: (56) if w2 != 0xa goto pc+14       ; R2_w=10
; p = bpf_sk_storage_get(&socket_cookies, ctx->sk, 0, @ socket_cookie_prog.c:40
4: (61) r2 = *(u32 *)(r1 +64)
invalid bpf_context access off=64 size=4 insn_idx=4 is_valid_access=c096ceec
processed 5 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
-- END PROG LOAD LOG --
libbpf: prog 'set_cookie': failed to load: -EACCES
libbpf: failed to load object 'socket_cookie_prog'
libbpf: failed to load BPF skeleton 'socket_cookie_prog': -EACCES
test_socket_cookie:FAIL:skel_open unexpected error: -13
\torvalds#366     socket_cookie:FAIL

Author: guidosarducci

net/core/filter.c

@@ -9268,7 +9268,7 @@ static bool sock_addr_is_valid_access(int off, int size,
 	case offsetof(struct bpf_sock_addr, sk):
 		if (type != BPF_READ)
 			return false;
-		if (size != sizeof(__u64))
+		if (size != sizeof(void *))
 			return false;
 		info->reg_type = PTR_TO_SOCKET;
 		break;