refactor ip6_getpcbopt() for better locking and memory management

[jason_eggnet.com]

Summary

Created GET_PKTOPT_EXT_HDR() and GET_PKTOPT_SOCKADDR() macros to
handle safely fetching options from in6p_outputopts, including
properly dealing with in6p locking and preparing memory for
sooptcopyout().

Changed the function signature of ip6_getpcbopt() to allow the
function to acquire and release locks on in6p as needed.

[melifaro]

head/sys/netinet6/ip6_output.c

2312

Why don't we declare this as function? It will be much easier to maintain.

2315

optdatalen = min(lenexpr, sopt->sopt_valsize) ? :-)

[jason_eggnet.com]

head/sys/netinet6/ip6_output.c

2315

yeah :)

[jason_eggnet.com]

fix malloc length in ip6_output.c:GET_PKTOPT_VAR

[jason_eggnet.com]

I will start a new review to address feedback from @melifaro, this one is broken.