sunrpc: Remove useless if (task == NULL) from xprt_reserve_xprt

The task in question is dereferenced above (and is actually never NULL).

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>