net: mark struct ip6_header as QEMU_PACKED

The ip6_header is often used by network devices to examine structures in
packet data, and it's not guaranteed to be aligned. This manifests as
errors from the clang sanitizer like this one:

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../../hw/net/rocker/rocker_of_dpa.c:321:37
../../hw/net/rocker/rocker_of_dpa.c:730:33: runtime error: member access within misaligned address 0x742970fe7ecd for type 'struct ip6_header', which requires 4 byte alignment
0x742970fe7ecd: note: pointer points here
 00 00 02 81 00 60 00  00 00 00 38 3a ff fe 80  00 00 00 00 00 00 00 00  00 00 00 00 00 02 ff 02  00
             ^

Fix this by marking the ip6_header struct as QEMU_PACKED, the way we
have done to handle similar problems involving tcp_header, ip_header,
etc.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>
Message-ID: <20260505201324.932323-1-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

include/net/eth.h

@@ -119,7 +119,7 @@ struct ip6_header {
     } ip6_ctlun;
     struct in6_address ip6_src;    /* source address */
     struct in6_address ip6_dst;    /* destination address */
-};
+} QEMU_PACKED;
 
 typedef struct ip6_pseudo_header {
     struct in6_address ip6_src;