sw test tool: Fix to compare OFPMatch using masks without byte boundary

Mask lengths without byte boundary are fixed as follows:

 - VLAN_VID, from 16 bits (2 bytes) to 12 + 1 bits
 - IPV6_EXTHDR, from 16 bits (2 bytes) to 9 bits

Reported-by: Arne Goetje <arne_goetje@accton.com>
Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>