NFC: hci: Split memcpy() of struct hcp_message flexible array

To work around a misbehavior of the compiler's ability to see into
composite flexible array structs (as detailed in the coming memcpy()
hardening series[1]), split the memcpy() of the header and the payload
so no false positive run-time overflow warning will be generated. This
split already existed for the "firstfrag" case, so just generalize the
logic further.

[1] https://lore.kernel.org/linux-hardening/20220901065914.1417829-2-keescook@chromium.org/

Cc: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Reported-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220924040835.3364912-1-keescook@chromium.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/nfc/hci/hcp.c

@@ -73,14 +73,12 @@ int nfc_hci_hcp_message_tx(struct nfc_hci_dev *hdev, u8 pipe,
 		if (firstfrag) {
 			firstfrag = false;
 			packet->message.header = HCP_HEADER(type, instruction);
-			if (ptr) {
-				memcpy(packet->message.data, ptr,
-				       data_link_len - 1);
-				ptr += data_link_len - 1;
-			}
 		} else {
-			memcpy(&packet->message, ptr, data_link_len);
-			ptr += data_link_len;
+			packet->message.header = *ptr++;
+		}
+		if (ptr) {
+			memcpy(packet->message.data, ptr, data_link_len - 1);
+			ptr += data_link_len - 1;
 		}
 
 		/* This is the last fragment, set the cb bit */