zed-functions.sh: escape newline to produce valid json

This was discovered when using Discords Slack compatible webhook.

Slack webhooks works without the escape, however Discord rightly refuses
the POST as it contains invalid JSON.

https://discord.com/developers/docs/resources/webhook#execute-slackcompatible-webhook

Valid (while escaping the newline:
```
+ msg_json='{"text": "*ZFS scrub_finish error for test on quartz*\nZFS has detected a data error:\n\n   eid: 124\n class: scrub_finish\n  host: quartz\n  time: \n error: \n objid: :\n  pool: test\n"}'
```

Invalid (no escape):
```
+ msg_json='{"text": "*ZFS scrub_finish error for test on quartz*
ZFS has detected a data error:\n\n   eid: 124\n class: scrub_finish\n  host: quartz\n  time: \n error: \n objid: :\n  pool: test\n"}'
```
The new line gets rendered and not sent inside the JSON as intended.

```
++ curl -X POST https://discord.com/api/webhooks/{webhook.id}/{webhook.token}/slack --header 'Content-Type: application/json' --data-binary '{"text": "*ZFS scrub_finish error for test on quartz*
ZFS has detected a data error:\n\n   eid: 124\n class: scrub_finish\n  host: quartz\n  time: \n error: \n objid: :\n  pool: test\n"}'
+ msg_out='{"message": "Cannot send an empty message", "code": 50006}'
```

Test method:
`root@quartz:/etc/zfs/zed.d# export ZED_ZEDLET_DIR=/etc/zfs/zed.d; export ZEVENT_EID=124; export ZEVENT_SUBCLASS=scrub_finish; export ZEVENT_POOL=test; export ZED_NOTIFY_DATA=1; bash -x ./data-notify.sh`

Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Philip Kauffman <philip@kauffman.me>
Closes #13049

cmd/zed/zed.d/zed-functions.sh

@@ -413,7 +413,7 @@ zed_notify_slack_webhook()
 
     # Construct the JSON message for posting.
     #
-    msg_json="$(printf '{"text": "*%s*\n%s"}' "${subject}" "${msg_body}" )"
+    msg_json="$(printf '{"text": "*%s*\\n%s"}' "${subject}" "${msg_body}" )"
 
     # Send the POST request and check for errors.
     #