If transmit lock is contended on, then push return code back
and retry at higher level.

Bugfix: If buffer is reallocated because of lack of headroom
and the send is blocked, then drop packet. This is necessary
because caller would end up requeuing a freed skb.
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>