e2fsck: support casefold directories when rehashing

e2fsck: support casefold directories when rehashing

When rehashing a +F directory, the casefold comparison needs to be
performed, in order to identify duplicated filenames.  Like the -F
version, This is done in two steps, first adapt the qsort comparison to
consider casefolded directories, and then iterate over the sorted list
fixing dups.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>