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

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.

Discussed with: bz (earlier revision)
Reviewed by: thj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D20011