bcache: Don't touch bucket gen for dirty ptrs

Unnecessary since a bucket that has dirty pointers pointing to it can
never be invalidated - and skipping it is a measurable performance
boost, since the bucket gen will usually be a cache miss.

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

drivers/md/bcache/request.c

@@ -630,7 +630,8 @@ static void bch_cache_read_endio(struct bio *bio, int error)
 
 	if (error)
 		s->iop.error = error;
-	else if (ptr_stale(s->iop.c, &b->key, 0)) {
+	else if (!KEY_DIRTY(&b->key) &&
+		 ptr_stale(s->iop.c, &b->key, 0)) {
 		atomic_long_inc(&s->iop.c->cache_read_races);
 		s->iop.error = -EINTR;
 	}