rc.d/tmp: Fix dupe mount check

Before mounting a new mfs on /tmp, we check if there already is one.
However, the dupe check only takes /dev/md[0-9]* into account, while the
default mfs type these days is tmpfs.  Rewrite it to look for tmpfs as
well.

Note that the dupe check is redundant in the tmpmfs=auto case, but we
leave moving it for later.

PR:		182035
MFC after:	1 week
Reviewed by:	kevans, allanjude
Differential Revision:	https://reviews.freebsd.org/D57682

libexec/rc/rc.d/tmp

@@ -42,10 +42,15 @@ tmp_svcj="NO"
 
 mount_tmpmfs()
 {
-	while read line; do
-		case $line in
-		/dev/md[0-9]*\ /tmp)
-			return;;
+	while read _dev _ _ _ _ _mp; do
+		case ${_mp} in
+		/tmp)
+			case ${_dev} in
+			tmpfs|/dev/md[0-9]*)
+				return 0
+				;;
+			esac
+			;;
 		esac
 	done <<*EOF
 $(df /tmp)