nilfs2: add missing set_freezable() for freezable kthread

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/20231219090918.2329-1-konishi.ryusuke@gmail.com
Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>