kernel: core_hook: call ksys_umount on 4.17+

Linux 4.17 added the following:

`fs: add ksys_umount() helper; remove in-kernel call to sys_umount()` torvalds/linux@3a18ef5
- which adds a ksys_umount helper, basically turning sys_umount

`syscalls/core: Introduce CONFIG_ARCH_HAS_SYSCALL_WRAPPER=y` torvalds/linux@1bd21c6
- which undefines in-kernel calls of syscalls, which is enabled on 4.19's arch/arm64/Kconfig

Ref:
- https://github.com/torvalds/linux/commits/v4.17/include/linux/syscalls.h
- rsuntk@d20f15e

Fixes: `kernel: core_hook: screw path_umount backport, call sys_umount directly` for 4.19/5.4

Tested-by: rsuntk <[email protected]>
Signed-off-by: backslashxx <[email protected]>

Author: rsuntk

kernel/core_hook.c

@@ -524,7 +524,11 @@ static void ksu_sys_umount(const char *mnt, int flags)
 
 	mm_segment_t old_fs = get_fs();
 	set_fs(KERNEL_DS);
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 17, 0)
+	int ret = ksys_umount(usermnt, flags);
+#else
 	long ret = sys_umount(usermnt, flags); // cuz asmlinkage long sys##name
+#endif
 	set_fs(old_fs);
 	pr_info("%s: path: %s code: %d \n", __func__, mnt, ret);
 }