bpf: Use 0 instead of NOT_INIT for btf_struct_access() writes

Returning a bpf_reg_type only makes sense in the context of a BPF_READ.
For writes, prefer to explicitly return 0 for clarity.

Note that is non-functional change as it just so happened that NOT_INIT
== 0.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/01772bc1455ae16600796ac78c6cc9fff34f95ff.1662568410.git.dxu@dxuuu.xyz
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/ipv4/bpf_tcp_ca.c

@@ -124,7 +124,7 @@ static int bpf_tcp_ca_btf_struct_access(struct bpf_verifier_log *log,
 		return -EACCES;
 	}
 
-	return NOT_INIT;
+	return 0;
 }
 
 BPF_CALL_2(bpf_tcp_send_ack, struct tcp_sock *, tp, u32, rcv_nxt)