Warn on leaked block refs on unmount

By the time we get to destroying the block cache we should have put all
our block references.  Warn as we tear down the blocks if we see any
blocks that still have references, implying a ref leak.  This caught a
leak caused by srch compaction forgetting to put allocator list block
refs.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/block.c

@@ -396,6 +396,7 @@ static void block_remove_all(struct super_block *sb)
 
 		if (block_get_if_inserted(bp)) {
 			block_remove(sb, bp);
+			WARN_ON_ONCE(atomic_read(&bp->refcount) != 1);
 			block_put(sb, bp);
 		}
 	}