ovl: fix oops in ovl_indexdir_cleanup() with nfs_export=on

Mounting with nfs_export=on, xfstests overlay/031 triggers a kernel panic
since v5.8-rc1 overlayfs updates.

 overlayfs: orphan index entry (index/00fb1..., ftype=4000, nlink=2)
 BUG: kernel NULL pointer dereference, address: 0000000000000030
 RIP: 0010:ovl_cleanup_and_whiteout+0x28/0x220 [overlay]

Bisect point at commit c21c839b

 ("ovl: whiteout inode sharing")

Minimal reproducer:
--------------------------------------------------
rm -rf l u w m
mkdir -p l u w m
mkdir -p l/testdir
touch l/testdir/testfile
mount -t overlay -o lowerdir=l,upperdir=u,workdir=w,nfs_export=on overlay m
echo 1 > m/testdir/testfile
umount m
rm -rf u/testdir
mount -t overlay -o lowerdir=l,upperdir=u,workdir=w,nfs_export=on overlay m
umount m
--------------------------------------------------

When mount with nfs_export=on, and fail to verify an orphan index, we're
cleaning this index from indexdir by calling ovl_cleanup_and_whiteout().
This dereferences ofs->workdir, that was earlier set to NULL.

The design was that ovl->workdir will point at ovl->indexdir, but we are
assigning ofs->indexdir to ofs->workdir only after ovl_indexdir_cleanup().
There is no reason not to do it sooner, because once we get success from
ofs->indexdir = ovl_workdir_create(... there is no turning back.

Reported-and-tested-by: Murphy Zhou <jencce.kernel@gmail.com>
Fixes: c21c839b

 ("ovl: whiteout inode sharing")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>