unicode: introduce test module for normalized utf8 implementation

This implements a in-kernel sanity test module for the utf8
normalization core.  At probe time, it will run basic sequences through
the utf8n core, to identify problems will equivalent sequences and
normalization/casefold code.  This is supposed to be useful for
regression testing when adding support for a new version of utf8 to
linux.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>