svc.c: Check for a non-NULL xp_socket

Commit a16ff32f04b5 added support to the kernel RPC to set
TCP_USE_DDP.
However, for the unusual case of a NFSv4.1/4.2 non-NULL callback,
the xp_socket field of SVCXPRT is NULL, since it uses the same
socket as the client->server connection.

This patch adds the check for this to avoid crashes.

This only affects NFSv4.1/4.2 mounts where either pNFS or
delegations are in use.

MFC after: 3 days