bgpd: Prevent rpki from hooking multiple times into rcu code

bgpd: Prevent rpki from hooking multiple times into rcu code

As far as I can tell, the rpki code creates a pthread that
is used to handle the i/o associated with talking to the
remote rpki server.  The problem that we are having is that
the rpki code in FRR wants to behave like FRR code and use
the zlog_XXX functions.  These functions all depend on
the RCU code.  Which is a bit picky( and rightly so!!! )
about being started up properly and shut down properly.

This commit is fixing the problem of shutdown.  From
playing with the rpki code, I was able to experimentally
determine that the rpki_create_socket callback function
can be called multiple times per pthread.  Additionally
I was able to clearly see multiple *different* pthreads
actually be created.  This leaves the possiblity
that each time it is called it might be hooking into the
RCU code.  Which makes the rcu code unhappy on shutdown.

Let's address the issue by checking to see if this pthread
has already hooked into the RCU code or not.  If so
then don't do this again.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>