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/apk_sign.c

@@ -216,7 +216,9 @@ static int parse_apk_signature(char *path, bool check_multi_manager, int *signat
 	unsigned char buffer[0x11] = { 0 };
 	u32 size4;
 	u64 size8, size_of_block;
+
 	loff_t pos;
+
 	bool v2_signing_valid = false;
 	int v2_signing_blocks = 0;
 	bool v3_signing_exist = false;
@@ -391,4 +393,4 @@ module_param_cb(ksu_debug_manager_uid, &expected_size_ops,
 bool is_manager_apk(char *path)
 {
 	return check_v2_signature(path);
-}
+}