udf: simplify __udf_read_inode

- move all brelse(ibh) after main if, because it's called
  on every path except one where ibh is null
- move variables to the most inner blocks

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>