ip6_randomflowlabel: Avoid blocking if random(4) is not available

[cem]

Summary

If kern.random.initial_seeding.bypass_before_seeding is disabled, random(4)
and arc4random(9) will block indefinitely until enough entropy is available
to initially seed Fortuna.

It seems that zero flowids are perfectly valid, so avoid blocking on random
until initial seeding takes place.

Test Plan

This is a reduced version of D19973 for just the flowlabels, which don't need
the hacked up RNG and can just emit zero early on.