The function is called a lot, I believe something like this would improve its performance in general.
-- v2: ntdll: Lookup extension from the end in hash_short_file_name. ntdll: Use invalid char lookup table in lookup_unix_name. ntdll: Use invalid char lookup table in nt_to_unix_file_name_no_root. ntdll: Use invalid char lookup table in is_legal_8dot3_name. ntdll: Use invalid char lookup table in is_invalid_dos_char.