move doc of userland physical address tests to README

kernel_module/user/virt_to_phys_test.c

@@ -0,0 +1,21 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#userland-physical-address-experiments */
+
+#define _XOPEN_SOURCE 700
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+enum { I0 = 0x12345678 };
+
+static volatile uint32_t i = I0;
+
+int main(void) {
+	printf("vaddr %p\n", (void *)&i);
+	printf("pid %ju\n", (uintmax_t)getpid());
+	while (i == I0) {
+		sleep(1);
+	}
+	printf("i %jx\n", (uintmax_t)i);
+	return EXIT_SUCCESS;
+}