dco: make key state desync recoverable
This was reported as a "with suitable timing, a key-update de-sync between
OpenVPN and the kernel could trigger an ASSERT()", and was initially
handled as security report. It turned out to be not exploitable, but the
state machine was not very robust and so the opportunity was used to
improve the code.
Bug found by 章鱼哥 (www.aipyaipy.com)
