topology-linux.c: ensure fd is marked as close-on-exec

Since this fd stays open for a while (i.e., after hwlow returns to the
application), make sure it is marked as close-on-exec so that if the
application forks/execs a child, it doesn't inherit this fd.

(cherry picked from commit 7489287ece8394191ed1454699dfe6df5e0d96ef)