apply eexist workaround from listDir to stat as well

Summary:
knetattach stats() without first doing a listDir() so in the event that
the url requires authentication it won't work and error out on the EEXISTS.
listDir has a fallback hack for this which was missing from stat.

stat now has this workaround as well and correctly brings up an auth
dialog on shares that require it.

BUG: 399699

Test Plan:

• knetattach
• smb
• data for host (folder should require auth)
• save and connect
• auth dialog comes up
• enter details
• ok
• works

Reviewers: broulik

Reviewed By: broulik

Subscribers: kde-frameworks-devel, kfm-devel

Tags: Dolphin, Frameworks

Differential Revision: https://phabricator.kde.org/D16272