Relax the block count check on deallocation in env_test

It seems that on some FS we get more blocks than we ask for. This is
already handled when checking the allocated number of blocks, but
after the file is closed it checks for an exact number of blocks,
which fails on my machine.

I changed the test to add one full page to the size, then calculate
the expected number of blocks and check if the actual number of blocks
is less or equal to that.

util/env_test.cc

@@ -552,6 +552,7 @@ TEST(EnvPosixTest, AllocateTest) {
   // allocate 100 MB
   size_t kPreallocateSize = 100 * 1024 * 1024;
   size_t kBlockSize = 512;
+  size_t kPageSize = 4096;
   std::string data(1024 * 1024, 'a');
   wfile->SetPreallocationBlockSize(kPreallocateSize);
   ASSERT_OK(wfile->Append(Slice(data)));
@@ -565,8 +566,7 @@ TEST(EnvPosixTest, AllocateTest) {
   // we only require that number of allocated blocks is at least what we expect.
   // It looks like some FS give us more blocks that we asked for. That's fine.
   // It might be worth investigating further.
-  auto st_blocks = f_stat.st_blocks;
-  ASSERT_LE((unsigned int)(kPreallocateSize / kBlockSize), st_blocks);
+  ASSERT_LE((unsigned int)(kPreallocateSize / kBlockSize), f_stat.st_blocks);
 
   // close the file, should deallocate the blocks
   wfile.reset();
@@ -574,7 +574,9 @@ TEST(EnvPosixTest, AllocateTest) {
   stat(fname.c_str(), &f_stat);
   ASSERT_EQ((unsigned int)data.size(), f_stat.st_size);
   // verify that preallocated blocks were deallocated on file close
-  ASSERT_EQ((f_stat.st_size + kBlockSize - 1) / kBlockSize, (unsigned int)f_stat.st_blocks);
+  // Because the FS might give us more blocks, we add a full page to the size
+  // and expect the number of blocks to be less or equal to that.
+  ASSERT_GE((f_stat.st_size + kPageSize + kBlockSize - 1) / kBlockSize, (unsigned int)f_stat.st_blocks);
 }
 #endif  // ROCKSDB_FALLOCATE_PRESENT