af_packet: check Tx error
af_packet: check Tx error

Coverity CID # 13200

If sendto fails, the packets will not get transmitted.  Return 0 as
the number of packets transmitted.

Signed-off-by: John W. Linville <2a53bac7a5d324865ef46ec4c38b2c0fba1456b4@tuxdriver.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk