MFC r319895:
Resurrect RTF_RNH_LOCKED flag and restore ability to call rtalloc1_fib() with acquired RIB lock. This fixes a possible panic due to trying to acquire RIB rlock when it is already exclusive locked. PR: [[ https://bugs.freebsd.org/215963 | 215963 ]], 215122 Sponsored by: Yandex LLC
Approved by: re (delphij)