smb: Make smb backend's "stat" call use smb311 posix extensions

Volker Lendeckerequested to merge
vlendec/gvfs:smb311-unix-extensions-1 into master

Together with the code in

https://gitlab.com/samba-team/samba/-/merge_requests/4080

improve the reliability of gvfs' stat call. Hopefully more to come later.

Signed-off-by: Volker Lendecke vl@samba.org

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