Split arpresolve() into fast/slow path.

This change isolates the most common case (e.g. successful lookup)

from more complicates scenarios. It also (tries to) make code
more simple by avoiding retry: cycle.

The actual goal is to prepare code to the upcoming change that will

allow LL address retrieval without acquiring LLE lock at all.

Reviewed by: ae
Differential Revision: https://reviews.freebsd.org/D3383