lguest example launcher truncates block device file to 0 length on problems

The function should also use ftruncate64() rather than ftruncate() to prevent
files over 4GB (not uncommon for a root filesystem) being zeroed.
Signed-off-by: Chris Malley <mail@chrismalley.co.uk>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/lguest/lguest.c

@@ -882,7 +882,7 @@ static u32 handle_block_output(int fd, const struct iovec *iov,
 		 * of the block file (possibly extending it). */
 		if (off + len > device_len) {
 			/* Trim it back to the correct length */
-			ftruncate(dev->fd, device_len);
+			ftruncate64(dev->fd, device_len);
 			/* Die, bad Guest, die. */
 			errx(1, "Write past end %llu+%u", off, len);
 		}