mkrelpath doesn't do what it should...

[David Miller <davem@davemloft.net>]

I was trying to figure out why the kernel image paths generated
automatically for me by grub-mkconfig were not correct.

I have /boot on a seperate partition, but in the generated config
files it uses paths like /boot/vmlinux-2632 etc.

The problem is grub-mkrelpath and it's usage in the scrips such as
"10_linux".

"/boot" is given to "grub-mkrelpath", and this results in
the identical "/boot" in rel_dirname.

So all the paths emitted by 10_linux end up being "/boot" based
instead of "/" based.

grub-mkrelpath seems to do the right thing if I pass it a full path,
f.e. giving it "/boot/vmlinux" emits the correct "/vmlinux"

Casually inspecting make_system_path_relative_to_its_root() shows what
appears to be another bug.  It seems to immediately break from the
loop when a different device number than "/"'s is seen, but what if I
have:

/one/two/three

Where / is /dev/hda1, /one/two is /dev/hda2 and /one/two/three is yet
another mount from /dev/hda3.  It seems like the premature loop exit
in this function will give us the wrong result here.

The loop needs to remember the string prefix point at which every
device number change occurs, and once the whole path has been
traversed it should unwind back to that point in order to emit
the result.