nilfs2: add missing set_freezable() for freezable kthread

The kernel thread function nilfs_segctor_thread() invokes the
try_to_freeze() in its loop. But all the kernel threads are non-freezable
by default. So if we want to make a kernel thread to be freezable, we
have to invoke set_freezable() explicitly.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Kevin Hao <[e-mail]>
Signed-off-by: Ryusuke Konishi <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/nilfs2/segment.c

···
 		   "segctord starting. Construction interval = %lu seconds, CP frequency < %lu seconds",
 		   sci->sc_interval / HZ, sci->sc_mjcp_freq / HZ);
 
+	set_freezable();
 	spin_lock(&sci->sc_state_lock);
  loop:
 	for (;;) {