SUNRPC: Don't return EPROTONOSUPPORT in svc_register()'s helpers

The RPC client returns -EPROTONOSUPPORT if there is a protocol version
mismatch (ie the remote RPC server doesn't support the RPC protocol
version sent by the client).

Helpers for the svc_register() function return -EPROTONOSUPPORT if they
don't recognize the passed-in IPPROTO_ value.

These are two entirely different failure modes.

Have the helpers return -ENOPROTOOPT instead of -EPROTONOSUPPORT.  This
will allow callers to determine more precisely what the underlying
problem is, and decide to report or recover appropriately.

This patch is part of a series that addresses
   http://bugzilla.kernel.org/show_bug.cgi?id=12256

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>