Linux: Remove ZFS_AC_KERNEL_SRC_MODULE_PARAM_CALL_CONST autotools check

On older kernels, the definition for `module_param_call()` typecasts
function pointers to `(void *)`, which triggers -Werror, causing the
check to return false when it should return true.

Fixing this breaks the build process on some older kernels because they
define a `__check_old_set_param()` function in their headers that checks
for a non-constified `->set()`. We workaround that through the c
preprocessor by defining `__check_old_set_param(set)` to `(set)`, which
prevents the build failures.

However, it is now apparent that all kernels that we support have
adopted the GRSecurity change, so there is no need to have an explicit
autotools check for it anymore. We therefore remove the autotools check,
while adding the workaround to our headers for the build time
non-constified `->set()` check done by older kernel headers.

Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>

Author: ryao

config/kernel-mod-param.m4

@@ -122,7 +122,6 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_FMODE_T
 	ZFS_AC_KERNEL_SRC_KUIDGID_T
 	ZFS_AC_KERNEL_SRC_KUID_HELPERS
-	ZFS_AC_KERNEL_SRC_MODULE_PARAM_CALL_CONST
 	ZFS_AC_KERNEL_SRC_RENAME
 	ZFS_AC_KERNEL_SRC_CURRENT_TIME
 	ZFS_AC_KERNEL_SRC_USERNS_CAPABILITIES
@@ -243,7 +242,6 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_FMODE_T
 	ZFS_AC_KERNEL_KUIDGID_T
 	ZFS_AC_KERNEL_KUID_HELPERS
-	ZFS_AC_KERNEL_MODULE_PARAM_CALL_CONST
 	ZFS_AC_KERNEL_RENAME
 	ZFS_AC_KERNEL_CURRENT_TIME
 	ZFS_AC_KERNEL_USERNS_CAPABILITIES

config/kernel.m4

@@ -30,12 +30,16 @@
 #include <linux/module.h>
 #include <linux/moduleparam.h>
 
-/* Grsecurity kernel API change */
-#ifdef MODULE_PARAM_CALL_CONST
+/*
+ * Despite constifying struct kernel_param_ops, some older kernels define a
+ * `__check_old_set_param()` function in their headers that checks for a
+ * non-constified `->set()`. This has long been fixed in Linux mainline, but
+ * since we support older kernels, we workaround it by using a preprocessor
+ * definition to disable it.
+ */
+#define	__check_old_set_param(set) (set)
+
 typedef const struct kernel_param zfs_kernel_param_t;
-#else
-typedef struct kernel_param zfs_kernel_param_t;
-#endif
 
 #define	ZMOD_RW 0644
 #define	ZMOD_RD 0444