tools: bpf: make use of reallocarray

reallocarray() is a safer variant of realloc which checks for
multiplication overflow in case of array allocation.  Since it's
not available in Glibc < 2.26 import kernel's overflow.h and
add a static inline implementation when needed.  Use feature
detection to probe for existence of reallocarray.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/build/feature/test-reallocarray.c

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <stdlib.h>
+
+int main(void)
+{
+	return !!reallocarray(NULL, 1, 1);
+}