Add cond_resched in block_free_work

I'm seeing consistent CPU soft lockups in block_free_work on
my bare metal system that aren't reached by VM instances. The
reason is that the bare metal machine has a ton more memory
available causing the block free work queue to grow much
larger in size, and then it has so much work that it can take 30+
seconds before it goes through it all.

This is all with a debug kernel. A non debug kernel will likely
zoom through the outstanding work here at a much faster rate.

Signed-off-by: Auke Kok <auke.kok@versity.com>

kmod/src/block.c

@@ -218,6 +218,7 @@ static void block_free_work(struct work_struct *work)
 
 	llist_for_each_entry_safe(bp, tmp, deleted, free_node) {
 		block_free(sb, bp);
+		cond_resched();
 	}
 }