bcache: Fix moving_gc deadlocking with a foreground write

Deadlock happened because a foreground write slept, waiting for a bucket
to be allocated. Normally the gc would mark buckets available for invalidation.
But the moving_gc was stuck waiting for outstanding writes to complete.
These writes used the bcache_wq, the same queue foreground writes used.

This fix gives moving_gc its own work queue, so it was still finish moving
even if foreground writes are stuck waiting for allocation. It also makes
work queue a parameter to the data_insert path, so moving_gc can use its
workqueue for writes.

Signed-off-by: Nicholas Swenson <nks@daterainc.com>
Signed-off-by: Kent Overstreet <kmo@daterainc.com>

drivers/md/bcache/bcache.h

@@ -628,6 +628,8 @@ struct cache_set {
 	/* Number of moving GC bios in flight */
 	struct semaphore	moving_in_flight;
 
+	struct workqueue_struct	*moving_gc_wq;
+
 	struct btree		*root;
 
 #ifdef CONFIG_BCACHE_DEBUG