rstp: Eliminate BPDU padding and uninitialized bytes.

rstp: Eliminate BPDU padding and uninitialized bytes.

When the RSTP implementation sent BPDUs, it failed to initialize some of
their bytes.  None of the code initialized an array of 7 padding bytes, and
some of it also failed to initialize the version1_length field.  In
addition, the padding bytes confused some implementations that did not
correctly ignore extra bytes.

This commit fixes both problems, by removing the padding bytes and
initializing every byte in outgoing messages.

Reported-by: David van Moolenbroek <dvmoolenbroek@aimvalley.nl>
Reported-at: https://mail.openvswitch.org/pipermail/ovs-discuss/2018-June/046864.html
Tested-by: David van Moolenbroek <dvmoolenbroek@aimvalley.nl>
Signed-off-by: Ben Pfaff <blp@ovn.org>