kern/partition: Limit recursion in part_iterate()

kern/partition: Limit recursion in part_iterate()

The part_iterate() is used by grub_partition_iterate() as a callback in
the partition iterate functions. However, part_iterate() may also call
the partition iterate functions which may lead to recursion. Fix potential
issue by limiting the recursion depth.

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