vswitchd: Avoid 100% CPU when secchan dies too many times.

vswitchd: Avoid 100% CPU when secchan dies too many times.

vswitchd restarts secchan when necessary, but it limits the maximum number
of tries to avoid wasting CPU when secchan repeatedly dies.  Unfortunately,
when this happens it also throws vswitchd into a busy-wait by calling
process_wait() on the dead secchan process, because it doesn't clear out
the process from the bridge structure.

This commit clears out the secchan process from the bridge structure, so
that we don't attempt to wait on it any longer, and should fix the busy-wait
problem.