igb: flush when in xmit_more mode and under descriptor pressure

Mirror the changes made to ixgbe in commit 2367a17390138f68b3aa28f2f220b8d7ff8d91f4
("ixgbe: flush when in xmit_more mode and under descriptor pressure")

Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc drivers/net/ethernet/intel/igb/igb_main.c
Simple merge