initrd: use do_mount() instead of ksys_mount()

All three calls to ksys_mount() in initrd-related kernel code can
be switched over to do_mount():
- the first and third arguments are const strings in the kernel,
  and do not need to be copied over from userspace;
- the fifth argument is NULL, and therefore no page needs to be,
  copied over from userspace;
- the second and fourth argument are passed through anyway.
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

init/do_mounts_initrd.c

@@ -54,7 +54,7 @@ static int init_linuxrc(struct subprocess_info *info, struct cred *new)
 	ksys_dup(0);
 	/* move initrd over / and chdir/chroot in initrd root */
 	ksys_chdir("/root");
-	ksys_mount(".", "/", NULL, MS_MOVE, NULL);
+	do_mount(".", "/", NULL, MS_MOVE, NULL);
 	ksys_chroot(".");
 	ksys_setsid();
 	return 0;
@@ -89,7 +89,7 @@ static void __init handle_initrd(void)
 	current->flags &= ~PF_FREEZER_SKIP;
 
 	/* move initrd to rootfs' /old */
-	ksys_mount("..", ".", NULL, MS_MOVE, NULL);
+	do_mount("..", ".", NULL, MS_MOVE, NULL);
 	/* switch root and cwd back to / of rootfs */
 	ksys_chroot("..");
 
@@ -103,7 +103,7 @@ static void __init handle_initrd(void)
 	mount_root();
 
 	printk(KERN_NOTICE "Trying to move old root to /initrd ... ");
-	error = ksys_mount("/old", "/root/initrd", NULL, MS_MOVE, NULL);
+	error = do_mount("/old", "/root/initrd", NULL, MS_MOVE, NULL);
 	if (!error)
 		printk("okay\n");
 	else {