sunrpc: Do not unregister services if not registered [BZ #5010]

The change in commit 71894681
has no effect because of two bugs which cancel each other out:
The svc_is_mapped condition is inverted, and svc_is_mapped
always returns false because the check is performed after
the service has already been unregistered.  As a result,
pmap_unset is called unconditionally, as before.