udf: add assertions

Add a few assertions into udf_discard_prealloc() to check that the file is
sane (mostly helps debugging further patches ;).

Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>