sunrpc: clear XPRT_SOCK_UPD_TIMEOUT when reset transport

Since transport->sock has been set to NULL during reset transport,
XPRT_SOCK_UPD_TIMEOUT also needs to be cleared. Otherwise, the
xs_tcp_set_socket_timeouts() may be triggered in xs_tcp_send_request()
to dereference the transport->sock that has been set to NULL.

Fixes: 7196dbb0 ("SUNRPC: Allow changing of the TCP timeout parameters on the fly")
Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
Signed-off-by: Liu Jian <liujian56@huawei.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>