ck_hs: Existence requires generation check.

It is possible for a defragmenting set or swap operation
to set a tombstone. If the probe sequence does not encounter
an empty slot and hits maximum write-side probe limit first
for it to fail to reprobe defragmenting store.

src/ck_hs.c

@@ -552,8 +552,6 @@ ck_hs_get(struct ck_hs *hs,
 		ck_pr_fence_load();
 
 		slot = ck_hs_map_probe(hs, map, &n_probes, &first, h, key, &object, probe);
-		if (slot == NULL)
-			return NULL;
 
 		ck_pr_fence_load();
 		g_p = ck_pr_load_uint(generation);