hash-funcs: tweak odering in devt_compare_func()

Let's order dev_t's by their major first, minor secondary. The binary
encoding of the two fields is weirdly interleaved and different in
kernel and glibc, hence let's focus on the generic part that works like
users would expect it.

So far the function is only used to compare for equality, not for
sorting, hence this has no immediate effect.

src/basic/hash-funcs.c

@@ -102,10 +102,16 @@ DEFINE_HASH_OPS(uint64_hash_ops, uint64_t, uint64_hash_func, uint64_compare_func
 void devt_hash_func(const dev_t *p, struct siphash *state) {
         siphash24_compress(p, sizeof(dev_t), state);
 }
+#endif
 
 int devt_compare_func(const dev_t *a, const dev_t *b) {
-        return CMP(*a, *b);
+        int r;
+
+        r = CMP(major(*a), major(*b));
+        if (r != 0)
+                return r;
+
+        return CMP(minor(*a), minor(*b));
 }
 
 DEFINE_HASH_OPS(devt_hash_ops, dev_t, devt_hash_func, devt_compare_func);
-#endif