Comment 5 for bug 1734129

This is basically how upstream packaging does it (with some additional restrictions: https://github.com/mysql/mysql-server/blob/5.7/packaging/deb-in/extra/mysql-helpers#L76); if the directory already exists it isn't touched.
There are some restrictions on what the permissions on the secure-file-priv location should be (the server will log a warning if it considers the directory insecure), but the proposed change will just make it more in line with the upstream packaging.

The patch needs an indentation fix, though :)