net: more accurate network taps in transmit path

dev_queue_xmit_nit() should be called right before ndo_start_xmit()
calls or we might give wrong packet contents to taps users :

Packet checksum can be changed, or packet can be linearized or
segmented, and segments partially sent for the later case.

Also a memory allocation can fail and packet never really hit the
driver entry point.

Reported-by: Jamie Gloudon <jamie.gloudon@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>