Btrfs: Fix block generation verification race

After the path is released, the generation number got from block
pointer is no long valid. The race may cause disk corruption, because
verify_parent_transid() calls clear_extent_buffer_uptodate() when
generation numbers mismatch.
Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.c

@@ -1604,7 +1604,7 @@ read_block_for_search(struct btrfs_trans_handle *trans,
 	btrfs_release_path(NULL, p);
 
 	ret = -EAGAIN;
-	tmp = read_tree_block(root, blocknr, blocksize, gen);
+	tmp = read_tree_block(root, blocknr, blocksize, 0);
 	if (tmp) {
 		/*
 		 * If the read above didn't mark this buffer up to date,