bcache: bch_writeback_thread() is not freezable

bch_writeback_thread() is calling try_to_freeze(), but that's just an
expensive no-op given the fact that the thread is not marked freezable.

I/O helper kthreads, exactly such as the bcache writeback thread, actually
shouldn't be freezable, because they are potentially necessary for
finalizing the image write-out.
Signed-off-by: NJiri Kosina <jkosina@suse.cz>
Signed-off-by: NJens Axboe <axboe@fb.com>

drivers/md/bcache/writeback.c

@@ -12,7 +12,6 @@
 #include "writeback.h"
 
 #include <linux/delay.h>
-#include <linux/freezer.h>
 #include <linux/kthread.h>
 #include <trace/events/bcache.h>
 
@@ -228,7 +227,6 @@ static void read_dirty(struct cached_dev *dc)
 	 */
 
 	while (!kthread_should_stop()) {
-		try_to_freeze();
 
 		w = bch_keybuf_next(&dc->writeback_keys);
 		if (!w)
@@ -433,7 +431,6 @@ static int bch_writeback_thread(void *arg)
 			if (kthread_should_stop())
 				return 0;
 
-			try_to_freeze();
 			schedule();
 			continue;
 		}