SUNRPC: Define xs_tcp_fin_timeout only if CONFIG_SUNRPC_DEBUG

Now that the linger code is gone, the xs_tcp_fin_timeout variable has
no real function. Keep it for now, since it is part of the /proc
interface, but only define it if that /proc interface is enabled.

Suggested-by: Anna Schumaker <Anna.Schumaker@netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>