lib/string_helpers: Change returned value of the strreplace()

It's more useful to return the pointer to the string itself
with strreplace(), so it may be used like

	attr->name = strreplace(name, '/', '_');

While at it, amend the kernel documentation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230605170553.7835-3-andriy.shevchenko@linux.intel.com