Fix icmpv6_reply logic broken by commit 48311cc61f and 391b501639.
- 48311cc61f "Fix unaligned memory access when creating ICMP Port Unreachable messages"
- It removed `htonl` function outside `data`, then the byte order may be wrong, so add `htons` back.
- 391b501639 "net: extract l3 header build code into new functions"
- It mis-removed the `memmove`, and the icmpv6 has no payload copied after this commit.
Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>