netinet6: simplify defrouter_select_fib()

• factor out underlying llentry check into a separate function and use it consistently
• enter epoch once instead of per-router enter/exit
• don't execute body with fibnum = RT_ALL_FIBS

Differential Revision: https://reviews.freebsd.org/D35523
MFC after: 2 weeks

(cherry picked from commit 9d16275c65bfe0f577e6f97397e024a33169acc9)