Currently we have 3 usage patterns:
1) nd6_output (most traffic flow, no lle supplied, lle RLOCK sufficient)
2) corner cases for output (no lle, STALE lle, so on). lle WLOCK needed.
3) nd* iunternal machinery (WLOCK'ed lle provided, perform packet queing).
We separate case 1 and implement it inside its only customer - nd6_output.
This leads to some code duplication (especialy SEND stuff, which should be
hooked to output in a different way), but simplifies locking and control
flow logic fir nd6_output_lle.
Reviewed by: ae
MFC after: 3 weeks
Sponsored by: Yandex LLC
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