btrfs: fix raid56 scrub failed in xfstests btrfs/072

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: Gui Hecheng <[email protected]>
Signed-off-by: Qu Wenruo <[email protected]>
Reviewed-by: Miao Xie <[email protected]>
Signed-off-by: Chris Mason <[email protected]>