passt: Address new clang-tidy warnings from LLVM 13.0.1

clang-tidy from LLVM 13.0.1 reports some new warnings from these
checkers:

- altera-unroll-loops, altera-id-dependent-backward-branch: ignore
  for the moment being, add a TODO item

- bugprone-easily-swappable-parameters: ignore, nothing to do about
  those

- readability-function-cognitive-complexity: ignore for the moment
  being, add a TODO item

- altera-struct-pack-align: ignore, alignment is forced in protocol
  headers

- concurrency-mt-unsafe: ignore for the moment being, add a TODO
  item

Fix bugprone-implicit-widening-of-multiplication-result warnings,
though, that's doable and they seem to make sense.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

util.h

@@ -144,9 +144,9 @@ enum {
 		.daddr		= IN6ADDR_ANY_INIT,			\
 	}
 
-#define RCVBUF_BIG		(2 * 1024 * 1024)
-#define SNDBUF_BIG		(4 * 1024 * 1024)
-#define SNDBUF_SMALL		(128 * 1024)
+#define RCVBUF_BIG		(2UL * 1024 * 1024)
+#define SNDBUF_BIG		(4UL * 1024 * 1024)
+#define SNDBUF_SMALL		(128UL * 1024)
 
 #include <net/if.h>
 #include <limits.h>