dm writecache: fix typo in error msg for creating writecache_flush_thread

The error msg should be "flush thread" instead of "endio thread"
for writecache_flush_thread.

Signed-off-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -2061,7 +2061,7 @@ invalid_optional:
 		if (IS_ERR(wc->flush_thread)) {
 			r = PTR_ERR(wc->flush_thread);
 			wc->flush_thread = NULL;
-			ti->error = "Couldn't spawn endio thread";
+			ti->error = "Couldn't spawn flush thread";
 			goto bad;
 		}
 		wake_up_process(wc->flush_thread);