netlink: fix error propagation in netlink_mmap()

Return the error if something went wrong instead of unconditionally
returning 0.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index d0b3dd6..57ee84d 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -371,7 +371,7 @@
 	err = 0;
 out:
 	mutex_unlock(&nlk->pg_vec_lock);
-	return 0;
+	return err;
 }
 
 static void netlink_frame_flush_dcache(const struct nl_mmap_hdr *hdr)