IB/mlx4: Convert is_xxx variables in build_mlx_header() to bool

Matches the way they're used, and actually lets at least x86-64 generate
better code:

    add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-38 (-38)
    function                                     old     new   delta
    mlx4_ib_post_send                           4416    4378     -38

Signed-off-by: Roland Dreier <roland@purestorage.com>
diff --git a/drivers/infiniband/hw/mlx4/qp.c b/drivers/infiniband/hw/mlx4/qp.c
index c6dde71..f8ac533 100644
--- a/drivers/infiniband/hw/mlx4/qp.c
+++ b/drivers/infiniband/hw/mlx4/qp.c
@@ -1746,11 +1746,11 @@
 	int header_size;
 	int spc;
 	int i;
-	int is_eth;
-	int is_vlan = 0;
-	int is_grh;
-	u16 vlan;
 	int err = 0;
+	u16 vlan;
+	bool is_eth;
+	bool is_vlan = false;
+	bool is_grh;
 
 	send_size = 0;
 	for (i = 0; i < wr->num_sge; ++i)