igb: change the head and tail offsets into pointers

Since we are writting to the head/tail pointers frequently we might as well
save ourselves some processing time by converting the head and tail offsets
directly to pointers.  This will shave a few cycles off the rx/tx path and
allows us to move one step closer to the rings being a bit more independant of
each other.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/igb/igb.h b/drivers/net/igb/igb.h
index 303df02..e52fee4 100644
--- a/drivers/net/igb/igb.h
+++ b/drivers/net/igb/igb.h
@@ -178,8 +178,8 @@
 	unsigned int count;            /* number of desc. in the ring */
 	u16 next_to_use;
 	u16 next_to_clean;
-	u16 head;
-	u16 tail;
+	void __iomem *head;
+	void __iomem *tail;
 	struct igb_buffer *buffer_info; /* array of buffer info structs */
 
 	u8 queue_index;