kern/disk: Limit recursion depth

kern/disk: Limit recursion depth

The grub_disk_read() may trigger other disk reads, e.g. via loopbacks.
This may lead to very deep recursion which can corrupt the heap. So, fix
the issue by limiting reads depth.

Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>