Rework e2fsck's dirinfo abstraction to be more friendly for databases

Rework e2fsck's dirinfo abstraction to be more friendly for databases

Change the iterator abstraction and replace e2fsck_get_dir_info() with
e2fsck_dir_info_{set,get}_{parent,dotdot} so that we can support an
on-disk dirinfo implementation.  This allows e2fsck to check very large
filesystems on systems with smaller amounts of memory and/or address
space.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>