btrfs: clear PF_NOFREEZE in cleaner_kthread()

cleaner_kthread() kthread calls try_to_freeze() at the beginning of every
cleanup attempt. This operation can't ever succeed though, as the kthread
hasn't marked itself as freezable.

Before (hopefully eventually) kthread freezing gets converted to fileystem
freezing, we'd rather mark cleaner_kthread() freezable (as my
understanding is that it can generate filesystem I/O during suspend).

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>