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/ksud.c

@@ -200,7 +200,7 @@ int ksu_handle_execveat_ksud(int *fd, struct filename **filename_ptr,
 			const char __user *p = get_user_arg_ptr(*argv, 1);
 			if (p && !IS_ERR(p)) {
 				char first_arg[16];
-				ksu_strncpy_from_user_nofault(
+				ksu_strncpy_from_user_retry(
 					first_arg, p, sizeof(first_arg));
 				pr_info("/system/bin/init first arg: %s\n",
 					first_arg);
@@ -225,7 +225,7 @@ int ksu_handle_execveat_ksud(int *fd, struct filename **filename_ptr,
 			const char __user *p = get_user_arg_ptr(*argv, 1);
 			if (p && !IS_ERR(p)) {
 				char first_arg[16];
-				ksu_strncpy_from_user_nofault(
+				ksu_strncpy_from_user_retry(
 					first_arg, p, sizeof(first_arg));
 				pr_info("/init first arg: %s\n", first_arg);
 				if (!strcmp(first_arg, "--second-stage")) {
@@ -250,7 +250,7 @@ int ksu_handle_execveat_ksud(int *fd, struct filename **filename_ptr,
 					}
 					char env[256];
 					// Reading environment variable strings from user space
-					if (ksu_strncpy_from_user_nofault(
+					if (ksu_strncpy_from_user_retry(
 						    env, p, sizeof(env)) < 0)
 						continue;
 					// Parsing environment variable names and values