Remove struct_rtentry_sz on FreeBSD

Summary:
Since struct rtentry is an internal kernel-only structure on FreeBSD,
and SIOCADDRT and SIOCDELRT are not supported anyway, stop including
socketvar.h and attempting to get at the definition of struct rtentry,
and move the line with struct_rtentry_sz to the SANIZER_LINUX block.

Reviewers: kcc, kutuzov.viktor.84, emaste

Reviewed By: kcc, emaste

Subscribers: emaste, llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D29832

llvm-svn: 294806