udf: Join functions for UTF8 and NLS conversions

There is no much sense to have separate functions for UTF8 and
NLS conversions, since UTF8 encoding is actually the special case
of NLS.

However, although UTF8 is also supported by general NLS framework,
it would be good to have separate UTF8 character conversion functions
(char2uni and uni2char) locally in UDF code, so that they could be
used even if NLS support is not enabled in the kernel configuration.

Signed-off-by: Andrew Gabbasov <andrew_gabbasov@mentor.com>
Signed-off-by: Jan Kara <jack@suse.cz>