SUNRPC: remove BUG_ON in svc_register

Instead of calling BUG_ON(), do a WARN_ON_ONCE() and return -EINVAL.

Signed-off-by: Weston Andros Adamson <dros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>