svcrpc: don't revert to SVC_POOL_DEFAULT on nfsd shutdown

This was unexpected behavior (at least for me)--why would you want
configuration settings automatically lost on nfsd restart?

In practice this won't affect distributions, which likely set everything
on every startup.  But I'd expect the behavior to be less confusing to
someone manually restarting nfsd for testing.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>