rxrpc: replace zero-lenth array with DECLARE_FLEX_ARRAY() helper

0-length arrays are deprecated, and cause problems with bounds checking.
Replace with a flexible array:

In file included from include/linux/string.h:253,
                 from include/linux/bitmap.h:11,
                 from include/linux/cpumask.h:12,
                 from arch/x86/include/asm/paravirt.h:17,
                 from arch/x86/include/asm/cpuid.h:62,
                 from arch/x86/include/asm/processor.h:19,
                 from arch/x86/include/asm/cpufeature.h:5,
                 from arch/x86/include/asm/thread_info.h:53,
                 from include/linux/thread_info.h:60,
                 from arch/x86/include/asm/preempt.h:9,
                 from include/linux/preempt.h:78,
                 from include/linux/percpu.h:6,
                 from include/linux/prandom.h:13,
                 from include/linux/random.h:153,
                 from include/linux/net.h:18,
                 from net/rxrpc/output.c:10:
In function 'fortify_memcpy_chk',
    inlined from 'rxrpc_fill_out_ack' at net/rxrpc/output.c:158:2:
include/linux/fortify-string.h:520:25: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()?  [-Werror=attribute-warning]
  520 |                         __write_overflow_field(p_size_field, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Link: https://lore.kernel.org/linux-next/20230105132535.0d65378f@canb.auug.org.au/
Cc: David Howells <dhowells@redhat.com>
Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: linux-afs@lists.infradead.org
Cc: netdev@vger.kernel.org
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Kees Cook <keescook@chromium.org>

net/rxrpc/ar-internal.h

@@ -783,7 +783,7 @@ struct rxrpc_txbuf {
 			u8	data[RXRPC_JUMBO_DATALEN]; /* Data packet */
 			struct {
 				struct rxrpc_ackpacket ack;
-				u8 acks[0];
+				DECLARE_FLEX_ARRAY(u8, acks);
 			};
 		};
 	} __aligned(64);