s3: smbd: Extract large directory case normalization code into a utility function...

s3: smbd: Extract large directory case normalization code into a utility function normalize_filename_case().

It is done before the original_lcomp is extracted
from the client pathname, so the new function that
will allow us to remove original_lcomp also needs
this logic.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>