udf: Adjust UDF_NAME_LEN to better reflect actual restrictions

Actual name length restriction is 254 bytes, this is used in 'ustr'
structure, and this is what fits into UDF File Ident structures.
And in most cases the constant is used as UDF_NAME_LEN-2.
So, it's better to just modify the constant to make it closer
to reality.

Also, in some cases it's useful to have a separate constant for
the maximum length of file name field in CS0 encoding in UDF File
Ident structures.

Also, remove the unused UDF_PATH_LEN constant.

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