bpf-restrict-fsaccess: move STAT_DEV_TO_KERNEL into generic code

We want to reuse it when processing sock-diag messages, hence let's
generalize this.

src/test/test-bpf-restrict-fsaccess.c

@@ -25,6 +25,7 @@
 #include <unistd.h>
 
 #include "bpf-restrict-fsaccess.h"
+#include "devnum-util.h"
 #include "fd-util.h"
 #include "log.h"
 #include "string-util.h"