sysfsutils: fix my_strncat function

sysfsutils: fix my_strncat function

The bug was introduced by upstream commit [1] where strncat was replaced with
internal my_strncat function, such as:

  char dest[32] = "/sys/devices/platform/axi";
  my_strncat(dest, "/", sizeof(dest) - strlen(dest) - 1);

Will result in dest string being:

  /sys/

and not the expected:

  /sys/devices/platform/axi/

The meaning of the "len" parameter in the my_strncat function is the size limit for
copying characters from "from", not the size limit for "to" after copying. Also,
the "#define safestrcat(to, from) my_strncat(to, from, sizeof(to) - strlen(to) - 1)"
has already imposed a limit on max based on the size of "to". Modify the function
to prevent truncation of content when too many bytes are passed to the my_strcat function.

[1] https://github.com/linux-ras/sysfsutils/commit/0719881cad85f837f039ecb378b823306640902a

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>