Make in_delayed_cksum() be similar to IPv6 implementation.

Use m_copyback() function to write checksum when it isn't located
in the first mbuf of the chain. Handmade analog doesn't handle the
case when parts of checksum are located in different mbufs.
Also in case when mbuf is too short, m_copyback() will allocate new
mbuf in the chain instead of making out of bounds write.

Also wrap long line and remove now useless KASSERTs.

X-MFC after: r334705