bcache: Kill bucket->gc_gen

gc_gen was a temporary used to recalculate last_gc, but since we only need
bucket->last_gc when gc isn't running (gc_mark_valid = 1), we can just update
last_gc directly.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

drivers/md/bcache/super.c

@@ -635,7 +635,7 @@ static void prio_read(struct cache *ca, uint64_t bucket)
 		}
 
 		b->prio = le16_to_cpu(d->prio);
-		b->gen = b->last_gc = b->gc_gen = d->gen;
+		b->gen = b->last_gc = d->gen;
 	}
 }