module: Clarify documentation of module_param_call()

Commit 9bbb9e5a ("param: use ops in struct kernel_param, rather than
get and set fns directly") added the comment that module_param_call()
was deprecated, during a large scale refactoring to bring sanity to type
casting back then. In 2017 following more cleanups, it became useful
again as it wraps a common pattern of creating an ops struct for a
given get/set pair:

  b2f270e8 ("module: Prepare to convert all module_param_call() prototypes")
  ece1996a ("module: Do not paper over type mismatches in module_param_call()")

        static const struct kernel_param_ops __param_ops_##name = \
                { .flags = 0, .set = _set, .get = _get }; \
        __module_param_call(MODULE_PARAM_PREFIX, \
                            name, &__param_ops_##name, arg, perm, -1, 0)

        __module_param_call(MODULE_PARAM_PREFIX, name, ops, arg, perm, -1, 0)

Many users of module_param_cb() appear to be almost universally
open-coding the same thing that module_param_call() does now. Don't
discourage[1] people from using module_param_call(): clarify the comment
to show that module_param_cb() is useful if you repeatedly use the same
pair of get/set functions.

[1] https://lore.kernel.org/lkml/202308301546.5C789E5EC@keescook/

Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Johan Hovold <johan@kernel.org>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Nick Desaulniers <ndesaulniers@gooogle.com>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Joe Perches <joe@perches.com>
Cc: linux-modules@vger.kernel.org
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

include/linux/moduleparam.h

@@ -293,7 +293,11 @@ struct kparam_array
 	= { __param_str_##name, THIS_MODULE, ops,			\
 	    VERIFY_OCTAL_PERMISSIONS(perm), level, flags, { arg } }
 
-/* Obsolete - use module_param_cb() */
+/*
+ * Useful for describing a set/get pair used only once (i.e. for this
+ * parameter). For repeated set/get pairs (i.e. the same struct
+ * kernel_param_ops), use module_param_cb() instead.
+ */
 #define module_param_call(name, _set, _get, arg, perm)			\
 	static const struct kernel_param_ops __param_ops_##name =	\
 		{ .flags = 0, .set = _set, .get = _get };		\