Fixed the signed/unsigned comparision error.

Signed-off-by: Ke Dong <kedong@google.com>

diff -Nura a/dm-bht_unittest.cc b/dm-bht_unittest.cc
--- a/dm-bht_unittest.cc	2012-05-03 18:59:08.000000000 -0700
+++ b/dm-bht_unittest.cc	2012-05-04 03:07:08.000000000 -0700
@@ -87,7 +87,7 @@
 
   int Read(sector_t start, u8 *dst, sector_t count) {
     EXPECT_LT(start, sectors_);
-    EXPECT_EQ(to_bytes(count), PAGE_SIZE);
+    EXPECT_EQ(to_bytes(count), (sector_t)PAGE_SIZE);
     u8 *src = &hash_data_[to_bytes(start)];
     memcpy(dst, src, to_bytes(count));
     return 0;