syslogd/tests: Improve loopback interface initialization

- In syslogd_start(), assign the lo0 address in the specified jail.
- Use the correct netmask.

Reviewed by:	jlduran
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D54799

usr.sbin/syslogd/tests/syslogd_test.sh

@@ -380,10 +380,8 @@ allowed_peer_test_setup()
     atf_check ifconfig ${epair}a vnet syslogd_allowed_peer
     atf_check ifconfig ${epair}b vnet syslogd_client
     atf_check jexec syslogd_allowed_peer ifconfig ${epair}a inet 169.254.0.1/16
-    atf_check jexec syslogd_allowed_peer ifconfig lo0 inet 127.0.0.1/8
     atf_check jexec syslogd_client ifconfig ${epair}b inet 169.254.0.2/16
     atf_check jexec syslogd_client ifconfig ${epair}b alias 169.254.0.3/16
-    atf_check jexec syslogd_client ifconfig lo0 inet 127.0.0.1/8
 }
 
 allowed_peer_test_cleanup()
@@ -529,12 +527,10 @@ forward_body()
     atf_check ifconfig ${epair}a vnet syslogd_server
     atf_check jexec syslogd_server ifconfig ${epair}a inet 169.254.0.1/16
     atf_check jexec syslogd_server ifconfig ${epair}a alias 169.254.0.2/16
-    atf_check jexec syslogd_server ifconfig lo0 inet 127.0.0.1/8
 
     syslogd_mkjail syslogd_client vnet
     atf_check ifconfig ${epair}b vnet syslogd_client
     atf_check jexec syslogd_client ifconfig ${epair}b inet 169.254.0.3/16
-    atf_check jexec syslogd_client ifconfig lo0 inet 127.0.0.1/8
 
     cat <<__EOF__ > ./client_config
 user.debug @169.254.0.1

usr.sbin/syslogd/tests/syslogd_test_common.sh

@@ -23,9 +23,6 @@ syslogd_start()
     local jail bind_arg conf_file pid_file socket privsocket
     local opt next other_args
 
-    # Setup loopback so we can deliver messages to ourself.
-    atf_check ifconfig lo0 inet 127.0.0.1/16
-
     OPTIND=1
     while getopts ":b:f:j:P:p:S:" opt; do
         case "${opt}" in
@@ -71,6 +68,11 @@ syslogd_start()
         esac
     done
 
+    # Setup loopback so we can deliver messages to ourself.
+    if [ $($jail sysctl -n security.jail.vnet) -ne 0 ]; then
+        atf_check $jail ifconfig lo0 inet 127.0.0.1/8
+    fi
+
     $jail syslogd \
         ${bind_arg:--b :${SYSLOGD_UDP_PORT}} \
         -C \