udf: Add read-only support for 2.50 UDF media

This patch implements parsing of metadata partitions and reading of Metadata
File thus allowing to read UDF 2.50 media. Error resilience is implemented
through accessing the Metadata Mirror File in case the data the Metadata File
cannot be read. The patch is based on the original patch by Sebastian Manciulea
<manciuleas@yahoo.com> and Mircea Fedoreanu <mirceaf_spl@yahoo.com>.

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