kernel: add wrapper for creds, refine disable_seccomp, revert some changes (#131)

1. Wrapper for creds:
* Some older kernel does not have {.val}, so, for nicer compatibility support and clean code,
make some wrapper for credential use.
* After this change, do not use current_uid().val, instead, use ksu_current_uid(). For more
info, check kernel/include/ksu_creds.h.

2. Refine disable_seccomp (need to add k6.11+ support)
https://github.com/tiann/KernelSU/pull/2708
https://github.com/tiann/KernelSU/issues/2706

3. Revert "Handle unmount for isolated process correctly"
Reason: https://github.com/tiann/KernelSU/pull/2696#issuecomment-3181866301

4. consolidate most of the gaps

Co-authored-by: Wang Han <416810799@qq.com>

kernel/manager.h

@@ -20,7 +20,7 @@ static inline bool ksu_is_manager_uid_valid()
 
 static inline bool is_manager()
 {
-	return unlikely(ksu_is_any_manager(current_uid().val) || ksu_manager_uid == current_uid().val);
+	return unlikely(ksu_is_any_manager(ksu_current_uid()) || ksu_manager_uid == ksu_current_uid());
 }
 
 static inline uid_t ksu_get_manager_uid()