ipsec gre: Don't cache bad ovs-monitor-ipsec pid values.

ipsec gre: Don't cache bad ovs-monitor-ipsec pid values.

Commit 2a586a5 (ipsec gre: Do not reread ovs monitor ipsec pidfile in
netdev vport so much) attempts to cache the pid of ovs-monitor-ipsec so
that it's not re-checked so often.  Unfortunately, it also cached error
returns, so errors never recover.  This commit continues to check for
the process's existence after an error.

Issue #12399

Reported-by: Paul Ingram <paul@nicira.com>
Signed-off-by: Justin Pettit <jpettit@nicira.com>