hotplug: Avoid race condition when creating or destroying network bridges

hotplug: Avoid race condition when creating or destroying network bridges

I saw the following message when I created or destroyed two bridges by
using network-bridge script at same time.  Of course names of the
bridges are different.  But, a temporal name "tmpbridge" is used by
the script to create or destroy the bridges.  I think that the message
was shown by "tmpbridge".

  SIOCSIFNAME: File exists

This patch avoids race condition when creating or destroying the
bridges.

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>