Clear ref_blkno output when block is already dirty

block_dirty_ref() skipped setting *ref_blkno when the block was
already dirty, leaving the caller with a stale value passed by
reference.

dirty_alloc_blocks() calls it twice but when the referenced block
is already dirty, it will receive the uninitialized stack value back,
and then adding it freed list with list_block_add() later.

Set it to 0 on the already-dirty fast path so callers do not try to
free a random block.

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

kmod/src/block.c

@@ -846,6 +846,8 @@ int scoutfs_block_dirty_ref(struct super_block *sb, struct scoutfs_alloc *alloc,
 		bp = BLOCK_PRIVATE(bl);
 
 		if (block_is_dirty(bp)) {
+			if (ref_blkno)
+				*ref_blkno = 0;
 			ret = 0;
 			goto out;
 		}