kern/51135: /dev/random blocks even with entropy available via callback

[pooka%iki.fi@localhost]

>Number:         51135
>Category:       kern
>Synopsis:       /dev/random blocks even with entropy available via callback
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri May 13 13:20:00 +0000 2016
>Originator:     Antti Kantee
>Release:        https://releng.netbsd.org/b5reports/i386/build/2016.05.13.05.45.13/test.html
>Organization:
>Environment:
>Description:
Reading from /dev/random blocks indefinitely even with entropy available from a randomness source via callback.  This problem makes for example "cgdconfig -k storedkey" hang.
>How-To-Repeat:
run tests/kernel/t_rnd::read_random

The test works both with netbsd-7 and -current from ~6 months ago.

>Fix: