Improve data directory write checking for NFS mounts

If `is_writable()` fails, fall back to logic that attempts to create a file
and then checks if it exists. If this check fails, an error occurs as it
did before.

Discussion on this solution was found here:
https://help.nextcloud.com/t/write-errors-for-nfs-mount/23328



Fixes #7124

Signed-off-by: Robert Dailey <rcdailey@gmail.com>