SketchBatchNumElements minor bugs

1) In xgboost/blob/master/src/common/hist_util.cu, SketchBatchNumElements,

auto avail = dh::AvailableMemory(device) * 0.8;
this is a **double**, and this function returns size_t.
return value, return **std::min**(sketch_batch_num_elements, kIntMax);

2) In xgboost/blob/master/tests/cpp/common/test_hist_util.cu, TEST(HistUtil, SketchBatchNumElements),

auto avail = static_cast(dh::AvailableMemory(device) * 0.8);
this is correct, not double. However, "avail_elem" isn't compared with KIntMax, which can be a very large integer. It causes the following code fails,

"ASSERT_EQ(batch, avail_elem);"

Thanks.

Inspect SketchBatchNumElements in src/common/hist_util.cu and the corresponding TEST(HistUtil, SketchBatchNumElements) in tests/cpp/common/test_hist_util.cu. Compare the available-memory calculation and the kIntMax handling in both places, then run the named test to confirm the expected batch value and that it passes.

Written by the indexing model from the issue text.