route/inet6: fix strncpy() in inet6_dump_details()

route/inet6: fix strncpy() in inet6_dump_details()

Compiler warnings:

  lib/route/link/inet6.c: In function ‘inet6_dump_details’:
  lib/route/link/inet6.c:383:3: error: ‘strncpy’ output may be truncated copying between 0 and 63 bytes from a string of length 63 [-Werror=stringop-truncation]
    383 |   strncpy(&buf[offset], buf2, strlen(buf2));
        |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Also, passing "strlen(buf2)" as length argument to strncpy() is
certainly wrong.