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

Co-authored-by: Wang Han <416810799@qq.com>
Co-authored-by: rsuntk <rsuntk@yukiprjkt.my.id>

kernel/Makefile

@@ -19,6 +19,7 @@ kernelsu-objs += selinux/rules.o
 
 ccflags-y += -I$(srctree)/security/selinux -I$(srctree)/security/selinux/include
 ccflags-y += -I$(objtree)/security/selinux -include $(srctree)/include/uapi/asm-generic/errno.h
+ccflags-y += -include $(srctree)/$(src)/include/ksu_creds.h
 
 obj-$(CONFIG_KSU) += kernelsu.o
 obj-$(CONFIG_KSU_TRACEPOINT_HOOK) += ksu_trace_export.o