bpf: Use -Wno-address-of-packed-member in some selftests

[Differences from V2:
- Remove conditionals in the source files pragmas, as the
  pragma is supported by both GCC and clang.]

Both GCC and clang implement the -Wno-address-of-packed-member
warning, which is enabled by -Wall, that warns about taking the
address of a packed struct field when it can lead to an "unaligned"
address.

This triggers the following errors (-Werror) when building three
particular BPF selftests with GCC:

  progs/test_cls_redirect.c
  986 |         if (ipv4_is_fragment((void *)&encap->ip)) {
  progs/test_cls_redirect_dynptr.c
  410 |         pkt_ipv4_checksum((void *)&encap_gre->ip);
  progs/test_cls_redirect.c
  521 |         pkt_ipv4_checksum((void *)&encap_gre->ip);
  progs/test_tc_tunnel.c
   232 |         set_ipv4_csum((void *)&h_outer.ip);

These warnings do not signal any real problem in the tests as far as I
can see.

This patch adds pragmas to these test files that inhibit the
-Waddress-of-packed-member warning.

Tested in bpf-next master.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240206102330.7113-1-jose.marchesi@oracle.com

tools/testing/selftests/bpf/progs/test_cls_redirect.c

@@ -22,6 +22,8 @@
 
 #include "test_cls_redirect.h"
 
+#pragma GCC diagnostic ignored "-Waddress-of-packed-member"
+
 #ifdef SUBPROGS
 #define INLINING __noinline
 #else

tools/testing/selftests/bpf/progs/test_cls_redirect_dynptr.c

@@ -23,6 +23,8 @@
 #include "test_cls_redirect.h"
 #include "bpf_kfuncs.h"
 
+#pragma GCC diagnostic ignored "-Waddress-of-packed-member"
+
 #define offsetofend(TYPE, MEMBER) \
 	(offsetof(TYPE, MEMBER) + sizeof((((TYPE *)0)->MEMBER)))
 

tools/testing/selftests/bpf/progs/test_tc_tunnel.c

@@ -20,6 +20,8 @@
 #include <bpf/bpf_endian.h>
 #include <bpf/bpf_helpers.h>
 
+#pragma GCC diagnostic ignored "-Waddress-of-packed-member"
+
 static const int cfg_port = 8000;
 
 static const int cfg_udp_src = 20000;