ixgb: Maybe stop TX if not enough free descriptors

A similar patch to commit 65c7973fa5b46b024f38be208aa477e8daf9a603
but now for ixgb.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
diff --git a/drivers/net/ixgb/ixgb.h b/drivers/net/ixgb/ixgb.h
index 50ffe90..f4aba43 100644
--- a/drivers/net/ixgb/ixgb.h
+++ b/drivers/net/ixgb/ixgb.h
@@ -171,6 +171,7 @@
 
 	/* TX */
 	struct ixgb_desc_ring tx_ring ____cacheline_aligned_in_smp;
+	unsigned int restart_queue;
 	unsigned long timeo_start;
 	uint32_t tx_cmd_type;
 	uint64_t hw_csum_tx_good;