init/do_mounts.c: create /root if it does not exist

init/do_mounts.c: create /root if it does not exist

If someone supplies an initramfs without /root in it, and we fail to
execute rdinit, we will try to mount root device and fail, for the mount
point does not exits.

But we get error message "VFS: Cannot open root device".  It's confusing.

We can give a more detailed error message, or we can go further: if /root
does not exit, create it.

Signed-off-by: YanHong <tempname2@hotmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Woody Suwalski <terraluna977@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>