kernel: Syncronize upstream changes (#198)

* Cherry-picked range: (kernel)
ebea31daa8..6915b62b9a

* Also merged unmerged pr:
https://github.com/tiann/KernelSU/pull/ 2909

Co-authored-by: Ylarod <me@ylarod.cn>
Co-authored-by: 5ec1cff <56485584+5ec1cff@users.noreply.github.com>
Co-authored-by: backslashxx <118538522+backslashxx@users.noreply.github.com>
Co-authored-by: u9521 <63995396+u9521@users.noreply.github.com>
Co-authored-by: Wang Han <416810799@qq.com>

kernel/kernel_umount.h

@@ -0,0 +1,12 @@
+#ifndef __KSU_H_KERNEL_UMOUNT
+#define __KSU_H_KERNEL_UMOUNT
+
+#include <linux/types.h>
+
+void ksu_kernel_umount_init(void);
+void ksu_kernel_umount_exit(void);
+
+// Handler function to be called from setresuid hook
+int ksu_handle_umount(uid_t old_uid, uid_t new_uid);
+
+#endif