udf: fix udf_debug macro

udf_debug should be enclosed with do { } while (0)
to be safely used in code like below:
if (something)
	udf_debug();
else
	anything;
(Otherwise compiler will not compile it with:
"error: expected expression before 'else'")

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>