svcrpc: fix rare race on unix_domain creation

Note that "new" here is not yet fully initialized; auth_domain_put
should be called only on auth_domains that have actually been added to
the hash.

Before this fix, two attempts to add the same domain at once could
cause the hlist_del in auth_domain_put to fail.

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