btrfs: fix raid56 scrub failed in xfstests btrfs/072

The xfstests btrfs/072 reports uncorrectable read errors in dmesg,
because scrub forgets to use commit_root for parity scrub routine
and scrub attempts to scrub those extents items whose contents are
not fully on disk.

To fix it, we just add the @search_commit_root flag back.
Signed-off-by: NGui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: NQu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: NMiao Xie <miaoxie@huawei.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/scrub.c

@@ -3065,6 +3065,8 @@ static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
 	path->search_commit_root = 1;
 	path->skip_locking = 1;
 
+	ppath->search_commit_root = 1;
+	ppath->skip_locking = 1;
 	/*
 	 * trigger the readahead for extent tree csum tree and wait for
 	 * completion. During readahead, the scrub is officially paused