dns: explicitly endian-convert all fields in header and question

For added type-safety and to avoid possible alignment issues make all
conversions explicit. While at it, mark header struct as packed so that
its layout is guaranteed to match RFC.

Prompted by gcc 8 & 9 warning in dns.c:

dns.c:261:2: error: converting a packed ‘struct dns_question’
pointer (alignment 1) to a ‘uint16_t’ {aka ‘short unsigned int’} pointer
(alignment 2) may result in an unaligned pointer value
[-Werror=address-of-packed-member]

261 |  uint16_t *swap = (uint16_t *) q;

Signed-off-by: Paul Fertser <fercerpav@gmail.com>
[Tweak commit message]
Acked-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

dns.c

@@ -237,16 +237,16 @@ static struct dns_header*
 dns_consume_header(uint8_t **data, int *len)
 {
 	struct dns_header *h = (struct dns_header *) *data;
-	uint16_t *swap = (uint16_t *) h;
-	int endianess = 6;
 
 	if (*len < sizeof(struct dns_header))
 		return NULL;
 
-	while (endianess--) {
-		*swap = be16_to_cpu(*swap);
-		swap++;
-	}
+	h->id = be16_to_cpu(h->id);
+	h->flags = be16_to_cpu(h->flags);
+	h->questions = be16_to_cpu(h->questions);
+	h->answers = be16_to_cpu(h->answers);
+	h->authority = be16_to_cpu(h->authority);
+	h->additional = be16_to_cpu(h->additional);
 
 	*len -= sizeof(struct dns_header);
 	*data += sizeof(struct dns_header);
@@ -258,16 +258,12 @@ static struct dns_question*
 dns_consume_question(uint8_t **data, int *len)
 {
 	struct dns_question *q = (struct dns_question *) *data;
-	uint16_t *swap = (uint16_t *) q;
-	int endianess = 2;
 
 	if (*len < sizeof(struct dns_question))
 		return NULL;
 
-	while (endianess--) {
-		*swap = be16_to_cpu(*swap);
-		swap++;
-	}
+	q->type = be16_to_cpu(q->type);
+	q->class = be16_to_cpu(q->class);
 
 	*len -= sizeof(struct dns_question);
 	*data += sizeof(struct dns_question);

dns.h

@@ -49,7 +49,7 @@ struct dns_header {
 	uint16_t answers;
 	uint16_t authority;
 	uint16_t additional;
-};
+} __attribute__((packed));
 
 struct dns_srv_data {
 	uint16_t priority;