When xmit_more mode is being used and the ring is about to
become full or the stack has stopped the ring, enforce a tail
pointer write to the hw. Otherwise, we could risk a TX hang.

Code suggested by Alexander Duyck.
Signed-off-by: NAlexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: NDaniel Borkmann <dborkman@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>