nfs: declare nfs_callback_tcp_port in header

Sparse warning:
fs/nfs/super.c:2638:16: sparse: symbol 'nfs_callback_tcpport' was not
declared. Should it be static?

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>