svcrpc: fix UDP on servers with lots of threads

commit b7e5034cbecf5a65b7bfdc2b20a8378039577706 upstream.

James Pearson found that an NFS server stopped responding to UDP
requests if started with more than 1017 threads.

sv_max_mesg is about 2^20, so that is probably where the calculation
performed by

	svc_sock_setbufsize(svsk->sk_sock,
                            (serv->sv_nrthreads+3) * serv->sv_max_mesg,
                            (serv->sv_nrthreads+3) * serv->sv_max_mesg);

starts to overflow an int.

Reported-by: James Pearson <jcpearson@gmail.com>
Tested-by: James Pearson <jcpearson@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>