nx: make pack string of IPv6 address consistent with OF1.2/1.3

Changed pack string of IPv6 address in NXM from former '!4I' to
'!8H'. Registered IPv6 src/dst field header so we could parse IPv6
addresses now.

Signed-off-by: Can Zhang <can@canx.me>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
nx: match_tuple() function now handles nw src/dst

match_tuple() function now handles nw src/dst correctly. FIX:
match_tuple function now handles the nw src/dst masks correctly. If
the mask is not valid, because there is no NX, it will be ignored.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>