Fix direct route installation with net/bird.

Slighly relax the gateway validation rules imposed by the
2fe5a79425c7, by requiring only first 8 bytes (everyhing
before sdl_data to be present in the AF_LINK gateway.

Reported by: olivier
PR: 255089

(cherry picked from commit 7f5f3fcc32bfa553faa007579dfcaed84be3b047)