OPP: fix error checking in opp_migrate_dentry()

Since commit ff9fb72b ("debugfs: return error values, not NULL")
changed return value of debugfs_rename() in error cases from %NULL to
%ERR_PTR(-ERROR), we should also check error values instead of NULL.

Fixes: ff9fb72b ("debugfs: return error values, not NULL")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Link: https://lore.kernel.org/r/20230208040037.60305-1-zhengqi.arch@bytedance.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>