Panic in arptimer on r255764

[Tim Kientzle]

I'm seeing this panic pretty consistently when I try to do a buildworld on r255764 (i386):

http://people.freebsd.org/~kientzle/r255764%20panic%202013-09-21%20at%209.27.09%20PM.png

I'm not seeing it on r255602, so I suspect it's a recent problem.

Running on VMWare Fusion 6.  This was about as vanilla a build as you can get:  no local code changes, no custom config, no src.conf or make.conf, no ports installed, etc.

Tim