udf: Make s_block_bitmap standard array

struct udf_bitmap has array of buffer pointers attached to it. The code
unnecessarily used s_block_bitmap as a pointer to the array instead of
the standard trick of using 0 length array in the declaration. Change
that to make code more readable and actually shrink the structure by one
pointer.
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/super.c

@@ -1023,7 +1023,6 @@ static struct udf_bitmap *udf_sb_alloc_bitmap(struct super_block *sb, u32 index)
 	if (bitmap == NULL)
 		return NULL;
 
-	bitmap->s_block_bitmap = (struct buffer_head **)(bitmap + 1);
 	bitmap->s_nr_groups = nr_groups;
 	return bitmap;
 }

fs/udf/udf_sb.h

@@ -83,7 +83,7 @@ struct udf_bitmap {
 	__u32			s_extLength;
 	__u32			s_extPosition;
 	int			s_nr_groups;
-	struct buffer_head 	**s_block_bitmap;
+	struct buffer_head 	*s_block_bitmap[0];
 };
 
 struct udf_part_map {