kernel-netlink: Let only a single thread work on a specific policy

kernel-netlink: Let only a single thread work on a specific policy

Other threads are free to add/update/delete other policies.

This tries to prevent race conditions caused by releasing the mutex while
sending messages to the kernel.  For instance, if break-before-make
reauthentication is used and one thread on the responder is delayed in
deleting the policies that another thread is concurrently adding for the
new SA.  This could have resulted in no policies being installed
eventually.

Fixes #1400.