Fixes 3675
It is possible for ipsec.InitSARemote to be called after ipsec.Destroy.
If an xfrm policy for that source/destination pair exists it will be
updated to the SPI from the Destroyed connection causing IPSEC to stop
working.
Added a check to make sure the xfrm policy has not yet been updated to
a new spi. The ipsec mutex is held at this point so nothing else should
be modifying this policy.
Hendrik van Wyk