In the UDP over IPv6 implementation several cases are using the wrong protocol,

In the UDP over IPv6 implementation several cases are using the wrong protocol,
e.g., based on wrong "next header" assumptions (which does not have to point to
the upper layer protocol), or using hard-coded UDP instead of UDP or UDP-Lite
possibly switching protocols.  Fix those cases for UDP-Lite to work correctly.

PR:			202788
Submitted by:		Tiwei Bie (btw mail.ustc.edu.cn) [parts]
Reviewed by:		gnn, Tiwei Bie (btw mail.ustc.edu.cn),
			kevlo (earlier version)
MFC after:		2 weeks
Differential Revision:	https://reviews.freebsd.org/D3686