lib/utils: naive refactoring of kr_dname_lf()

It's not optimal; we can improve performance later.

lib/cache/impl.h

@@ -115,6 +115,8 @@ struct key {
 	/** The key data start at buf+1, and buf[0] contains some length.
 	 * For details see key_exact* and key_NSEC* functions. */
 	uint8_t buf[KR_CACHE_KEY_MAXLEN];
+	/* LATER(opt.): ^^ probably change the anchoring, so that kr_dname_lf()
+	 * doesn't need to move data after knot_dname_lf(). */
 };
 
 static inline size_t key_nwz_off(const struct key *k)

lib/utils.h

@@ -420,12 +420,14 @@ int knot_dname_lf2wire(knot_dname_t *dst, uint8_t len, const uint8_t *lf);
  */
 static inline int kr_dname_lf(uint8_t *dst, const knot_dname_t *src, bool add_wildcard)
 {
-	int ret = knot_dname_lf(dst, src, NULL);
-	if (ret)
-		return ret;
-	int len = dst[0];
-	if (len == 1)
-		len = 0;
+	knot_dname_storage_t right_aligned_dst;
+	uint8_t *right_aligned_dname_start = knot_dname_lf(src, right_aligned_dst);
+	if (!right_aligned_dname_start) {
+		return kr_error(EINVAL);
+	}
+	int len = right_aligned_dname_start[0];
+	assert(right_aligned_dname_start + 1 + len - KNOT_DNAME_MAXLEN == right_aligned_dst);
+	memcpy(dst + 1, right_aligned_dname_start + 1, len);
 	if (add_wildcard) {
 		if (len + 2 > KNOT_DNAME_MAXLEN)
 			return kr_error(ENOSPC);