Add sunrpc compat define for xp_sock.

SunRPC is using xp_sock in SVCXPRT, while TI-RPC is using
xp_fd. Add a compatibility define.

Illumos has something similar for the non-kernel case.

Obtained from: linux-nfs project (git 0d94036c3a0d4c24d22bf6a8c40ac6625d972c29)