After a "bad collections", avoid switching back back to generational

After a major bad collection (one that collects too few objects),
next collection will be major again. In that case, avoid switching
back to generational mode (as it will have to switch again to
incremental to do next major collection).

lstate.h

@@ -193,6 +193,7 @@ typedef struct global_State {
   l_mem totalbytes;  /* number of bytes currently allocated - GCdebt */
   l_mem GCdebt;  /* bytes allocated not yet compensated by the collector */
   lu_mem GCestimate;  /* an estimate of the non-garbage memory in use */
+  lu_mem lastatomic;  /* see function 'genstep' in file 'lgc.c' */
   stringtable strt;  /* hash table for strings */
   TValue l_registry;
   TValue nilvalue;  /* a nil value */