ext2fs: implement faster CI comparison of strings

ext2fs: implement faster CI comparison of strings

Instead of calling casefold two times and memcmp the result, which
require allocating a temporary buffer for the casefolded version, add a
strcasecmp-like method to perform the comparison of each code-point
during the casefold itself.

This method is exposed because it needs to be used directly by fsck.

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>