init: add an init_chroot helper

Add a simple helper to chroot with a kernel space file name and switch
the early init code over to it.  Remove the now unused ksys_chroot.

Signed-off-by: Christoph Hellwig <hch@lst.de>
diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
index e48aaba..eac184e 100644
--- a/drivers/base/devtmpfs.c
+++ b/drivers/base/devtmpfs.c
@@ -413,7 +413,7 @@ static int __init devtmpfs_setup(void *p)
 	if (err)
 		goto out;
 	init_chdir("/.."); /* will traverse into overmounted root */
-	ksys_chroot(".");
+	init_chroot(".");
 out:
 	*(int *)p = err;
 	complete(&setup_done);