Allow dns query to fail when set "spray" option.

[yuguorui]

When the "spray" parameter is set, dnsredir will retry indefinitely
when it encounters an error. When the network is temporarily
interrupted or misconfigured, dnsredir will eat all the CPU.

This patch fixes this issue.

Signed-off-by: yuguorui yuguorui96@gmail.com

[yuguorui]

How to reproduce:

1. Create a coredns instance with this Corefile:

.:5354 {
        dnsredir . {
                spray
                to tls://127.0.0.1 # configured as an invalid address intentionally
        }
}

2. query the server with this cli.

nslookup -port=5354 baidu.com 127.0.0.1

[leiless]

How to reproduce:

1. Create a coredns instance with this Corefile:

.:5354 {
        dnsredir . {
                spray
                to tls://127.0.0.1 # configured as an invalid address intentionally
        }
}

2. query the server with this cli.

nslookup -port=5354 baidu.com 127.0.0.1

Thank you! I can also reproduce this bug.