Fix race between first rand(3) calls with _once().

Before this patch there was a chance for thread that called rand(3)
slightly later to see rand3_state already allocated, but not yet
initialized. While this API is not expected to be thread-safe, it
is not expected to crash. ztest on 64-thread system reproduced it
reliably for me.

Submitted by: avg@
MFC after: 1 month