kernel: selinux: fix pointer mismatch with 32-bit ksud on 64-bit kernels

Since KernelSU Manager can now be built for 32-bit, theres this problematic
setup where userspace is 32-bit (armeabi-v7a) and kernel is 64bit (aarch64).

On 64-bit kernels with CONFIG_COMPAT=y, 32-bit userspace passes 32-bit pointers.
These values are interpreted as 64-bit pointers without proper casting and that
results in invalid or near-null memory access.

This patch adds proper compat-mode handling with the ff changes:
- introduce a dedicated struct (`sepol_compat_data`) using u32 fields
- use `compat_ptr()` to safely convert 32-bit user pointers to kernel pointers
- adding a runtime `ksu_is_compat` flag to dynamically select between struct layouts

This prevents a near-null pointer dereference when handling SELinux
policy updates from 32-bit ksud in a 64-bit kernel.

Truth table:

kernel 32 + ksud 32, struct is u32, no compat_ptr
kernel 64 + ksud 32, struct is u32, yes compat_ptr
kernel 64 + ksud 64, struct is u64, no compat_ptr

Preprocessor check

64BIT=y COMPAT=y: define both structs, select dynamically
64BIT=y COMPAT=n: struct u64
64BIT=n: struct u32

Signed-off-by: backslashxx <118538522+backslashxx@users.noreply.github.com>

kernel/ksud.c

@@ -27,8 +27,6 @@
 #include "kernel_compat.h"
 #include "selinux/selinux.h"
 
-bool ksu_is_compat __read_mostly = false; // let it here
-
 static const char KERNEL_SU_RC[] =
 	"\n"