random(4): Squash non-error timeout codes from tsleep(9).

In both scenarios a timeout (EWOULDBLOCK) is considered as a
normal condition and the error should not pop up to upper layers.

PR: 231181
Submitted by: cem
Reported by: lev
Reviewed by: vangyzen, markm, delphij
Approved by: re (kib)
Approved by: secteam (delphij)
Differential Revision: https://reviews.freebsd.org/D17049