af_packet driver needs to check VLIB_BUFFER_NEXT_PRESENT flag is set when walking  vlib_buffer_t next_buffer chain on transmit. On buffer allocation: - next_buffer is not and may contain a stale invalid value that   should be ignored if not overwritten by a valid value. - VLIB_BUFFER_NEXT_PRESENT flag is cleared and only set   if a valid value is written to next_buffer.
af_packet driver needs to check VLIB_BUFFER_NEXT_PRESENT flag is set
when walking  vlib_buffer_t next_buffer chain on transmit.
On buffer allocation:
- next_buffer is not and may contain a stale invalid value that
  should be ignored if not overwritten by a valid value.
- VLIB_BUFFER_NEXT_PRESENT flag is cleared and only set
  if a valid value is written to next_buffer.

Change-Id: I9b0ccdc54f4f7456f8328ce7c4a0d52d0fba8caa
Signed-off-by: Jim Gibson <07a071fc54e0d7ee8b73adc4dac0d80a44269ae6@cisco.com>

milanlenco/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,milanlenco/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,milanlenco/vpp,vpp-dev/vpp,vpp-dev/vpp,milanlenco/vpp,chrisy/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,milanlenco/vpp,vpp-dev/vpp,milanlenco/vpp,FDio/vpp