bcachefs: Improve btree_key_cache_flush_pos()

btree_key_cache_flush_pos() uses BTREE_ITER_CACHED_NOFILL - but it
wasn't checking for !ck->valid. It does check for the entry being dirty,
so it shouldn't matter, but this refactor it a bit and adds and
assertion.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_key_cache.c

@@ -390,16 +390,20 @@ static int btree_key_cache_flush_pos(struct btree_trans *trans,
 		goto out;
 
 	ck = (void *) c_iter.path->l[0].b;
-	if (!ck ||
-	    (journal_seq && ck->journal.seq != journal_seq))
+	if (!ck)
 		goto out;
 
 	if (!test_bit(BKEY_CACHED_DIRTY, &ck->flags)) {
-		if (!evict)
-			goto out;
-		goto evict;
+		if (evict)
+			goto evict;
+		goto out;
 	}
 
+	BUG_ON(!ck->valid);
+
+	if (journal_seq && ck->journal.seq != journal_seq)
+		goto out;
+
 	/*
 	 * Since journal reclaim depends on us making progress here, and the
 	 * allocator/copygc depend on journal reclaim making progress, we need