tg3: Add tx BD budgeting code

tg3: Add tx BD budgeting code

As the driver breaks large skb fragments into smaller submissions to the
hardware, there is a new danger that BDs might get exhausted before all
fragments have been mapped.  This patch adds code to make sure tx BDs
aren't oversubscribed and flag the condition if it happens.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 84b67b27e9531e9a70c9e8cd952d66c55f4d0ddb)

Signed-off-by: Joe Jin <joe.jin@oracle.com>