bcache: Split out sort_extent_cmp()

Only use extent comparison for comparing extents, so we're not using
START_KEY() on other key types (i.e. btree pointers)

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 8aaaf16..e1e36e7 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1854,10 +1854,16 @@
 
 	while (1) {
 		struct bkey *k = bch_btree_iter_next(iter);
-		if (!k ||
-		    bkey_cmp(&START_KEY(k), insert) >= 0)
+		if (!k)
 			break;
 
+		if (bkey_cmp(&START_KEY(k), insert) >= 0) {
+			if (KEY_SIZE(k))
+				break;
+			else
+				continue;
+		}
+
 		if (bkey_cmp(k, &START_KEY(insert)) <= 0)
 			continue;