udf: use get_bh()

Make UDF use get_bh() instead of directly accessing b_count and use
brelse() instead of udf_release_data() which does just brelse()...

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>