Check input buffer size in zisofs

This uses the new deflateBound() thing to sanity-check the input to the
zlib decompressor before we even bother to start reading in the blocks.

Problem noted by Tim Yamin <[e-mail]>

fs/isofs/compress.c

···
 	cend = le32_to_cpu(*(__le32 *)(bh->b_data + (blockendptr & bufmask)));
 	brelse(bh);
 
+	if (cstart > cend)
+		goto eio;
+		
 	csize = cend-cstart;
+
+	if (csize > deflateBound(1UL << zisofs_block_shift))
+		goto eio;
 
 	/* Now page[] contains an array of pages, any of which can be NULL,
 	   and the locks on which we hold.  We should now read the data and