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

usr.sbin/autofs/common.c

@@ -140,7 +140,7 @@ create_directory(const char *path)
 	 */
 	copy = tofree = checked_strdup(path + 1);
 
-	partial = checked_strdup("");
+	partial = checked_strdup("/");
 	for (;;) {
 		component = strsep(&copy, "/");
 		if (component == NULL)