ovs-lib: Add timeout at ovs-check-dead-ifs.

ovs-lib: Add timeout at ovs-check-dead-ifs.

At SUSE12 SP3, we hit a case where ovs-check-dead-ifs tries to read
an entry in /proc/<pid>/fd/<some fd> but hangs forever.  The pid is
a qemu-system-x86_64 process and we suspect it's an issue related to
qemu, not ovs.  As a result, force-reload-kmod hangs and OVS bridge
never gets restarted. This patch adds a timeout of 5-seconds to
ovs-check-dead-ifs.

VMware-BZ: #2408059
Signed-off-by: William Tu <u9012063@gmail.com>
Cc: Ashish Varma <ashishvarma.ovs@gmail.com>
Cc: Gurucharan Shetty <guru@ovn.org>
Signed-off-by: Gurucharan Shetty <guru@ovn.org>