udf: Fix bug in VAT mapping code

Fix mapping of blocks using VAT when it is stored in an inode.
UDF_I(inode)->i_data already points to the beginning of VAT header so there's
no need to add udf_ext0_offset(inode).

Signed-off-by: Sebastian Manciulea <manciuleas@yahoo.com>
Signed-off-by: Jan Kara <jack@suse.cz>