linux: fix up SO_REUSEPORT back-port

Commit 3d2c820 back-ports a patch from the master branch that disables
the use of SO_REUSEPORT on Linux for reasons mentioned in the commit
log.

Unfortunately, the back-port was incomplete; another setsockopt() call
site in src/unix/udp.c was overlooked.  This commit rectifies that.

Hat tip to Luca Bruno for helping troubleshoot the issue.