HPA: Generalize purging.

Previously, we would purge a hugepage only when it's completely empty.  With
this change, we can purge even when only partially empty.  Although the
heuristic here is still fairly primitive, this infrastructure can scale to
become more advanced.

test/unit/hpdata.c

@@ -169,6 +169,7 @@ TEST_BEGIN(test_hugify) {
 	expect_false(hpdata_changing_state_get(&hpdata), "");
 	hpdata_hugify_begin(&hpdata);
 	expect_true(hpdata_changing_state_get(&hpdata), "");
+
 	hpdata_hugify_end(&hpdata);
 	expect_false(hpdata_changing_state_get(&hpdata), "");