automount(8): fix absolute path when creating a mountpoint

When executing automount(8), it will attempt to create the directory where an
autofs filesystem is to be mounted. Explicity set the root path for this
directory to "/".

This fixes the issue where the directory being created was being treated as a
relative path instead of an absolute path (as expected).

PR: 224601
Reported by: kusumi.tomohiro@gmail.com
Reviewed by: trasz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27832