dm log-writes: Add missing set_freezable() for freezable kthread

dm log-writes: Add missing set_freezable() for freezable kthread

The log_writes_kthread() calls try_to_freeze() but lacks set_freezable(),
rendering the freeze attempt ineffective since kernel threads are
non-freezable by default. This prevents proper thread suspension during
system suspend/hibernate.

Add set_freezable() to explicitly mark the thread as freezable.

Fixes: 0e9cebe72459 ("dm: add log writes target")
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>