jfs: Add missing set_freezable() for freezable kthread

The kernel thread function jfs_lazycommit() and jfs_sync() 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: Dave Kleikamp <dave.kleikamp@oracle.com>