bpf: Add a check for struct bpf_fib_lookup size

BugLink: https://bugs.launchpad.net/bugs/2076435

[ Upstream commit 59b418c7063d30e0a3e1f592d47df096db83185c ]

The struct bpf_fib_lookup should not grow outside of its 64 bytes.
Add a static assert to validate this.

Suggested-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240326101742.17421-4-aspsk@isovalent.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Portia Stephens <portia.stephens@canonical.com>
Signed-off-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>

net/core/filter.c

@@ -87,6 +87,9 @@
 
 #include "dev.h"
 
+/* Keep the struct bpf_fib_lookup small so that it fits into a cacheline */
+static_assert(sizeof(struct bpf_fib_lookup) == 64, "struct bpf_fib_lookup size check");
+
 static const struct bpf_func_proto *
 bpf_sk_base_func_proto(enum bpf_func_id func_id);