Refactor ip6_getpcbopt() for better locking and memory management

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.

Submitted by: Jason Eggleston <jason@eggnet.com>
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D14619