checkpath: fix initial dirfd opening

dirfd is uninitialized at this point, and even if it were, it doesn't
make sense to use since the path is "/" -- the dirfd is ignored when
the path is absolute.  Switch to AT_FDCWD to avoid all that.

src/checkpath/checkpath.c

@@ -89,7 +89,7 @@ static int get_dirfd(char *path, bool symlinks)
 
 	if (!path || *path != '/')
 		eerrorx("%s: empty or relative path", applet);
-	dirfd = openat(dirfd, "/", O_RDONLY);
+	dirfd = openat(AT_FDCWD, "/", O_RDONLY);
 	if (dirfd == -1)
 		eerrorx("%s: unable to open the root directory: %s",
 				applet, strerror(errno));