Revert "dm cache: do not wake_worker() in free_migration()"

This reverts commit 386cb7cd.

Taking the wake_worker() out of free_migration() will slow writeback
dramatically, and hence adaptability.

Say we have 10k blocks that need writing back, but are only able to
issue 5 concurrently due to the migration bandwidth: it's imperative
that we wake_worker() immediately after migration completion; waiting
for the next 1 second wake up (via do_waker) means it'll take a long
time to write that all back.
Reported-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-target.c

@@ -424,6 +424,7 @@ static void free_migration(struct dm_cache_migration *mg)
 		wake_up(&cache->migration_wait);
 
 	mempool_free(mg, cache->migration_pool);
+	wake_worker(cache);
 }
 
 static int prealloc_data_structs(struct cache *cache, struct prealloc *p)