RSTP testing - PATCH: BPDU length calculation

RSTP testing - PATCH: BPDU length calculation

   The bpdu_len sent to STP_OUT_tx_bpdu() excludes the ethernet and
   and LLC headers, though the bpdu pointer points to the start of the
   ethernet header. Given the types in rstplib/stp_bpdu.h, we need to
   be sending a packet with (base, len) given by
   (bpdu, bpdu_len + sizeof(MAC_HEADER_T) + sizeof(ETH_HEADER_T))
   MAC_HEADER_T is dest and source ethernet addresses and ETH_HEADER_T
   is the 802.2 part.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>