kernel-pfkey: Clear receive buffer before sending request

kernel-pfkey: Clear receive buffer before sending request

Many of the messages sent by the kernel, including confirmations to our
requests, are sent as broadcasts to all PF_KEY sockets.  So if an
external tool is used to manage SAs/policies (e.g. unrelated to IPsec)
the receive buffer might be filled, resulting in errors like these:

  error sending to PF_KEY socket: No buffer space available

To avoid this, just clear the buffer before sending any message.

Fixes #3225.