gfs2: Add missing set_freezable() for freezable kthread

The kernel thread function gfs2_logd() and gfs2_quotad() invoke the
try_to_freeze() in its loop. But all the kernel threads are no-freezable
by default. So if we want to make a kernel thread to be freezable,
we have to invoke set_freezable() explicitly.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>