Make fio -Wshadow clean

Found a few issues, actually.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/engines/e4defrag.c b/engines/e4defrag.c
index 6854070..d6113a9 100644
--- a/engines/e4defrag.c
+++ b/engines/e4defrag.c
@@ -98,8 +98,8 @@
 	}
 
 	if (!o->inplace) {
-		long long len = td->o.file_size_high - td->o.start_offset;
-		r = fallocate(ed->donor_fd, 0, td->o.start_offset, len);
+		long long __len = td->o.file_size_high - td->o.start_offset;
+		r = fallocate(ed->donor_fd, 0, td->o.start_offset, __len);
 		if (r)
 			goto err;
 	}