Before this change operations.SetDirModTime could return the error
"optional feature not implemented" when attempting to set modification
times on crypted sftp backends.
This was because crypt wraps the directories using fs.DirWrapper but
these return fs.ErrorNotImplemented for the SetModTime method.
The fix is to recognise that error and fall back to using the
DirSetModTime method on the backend which does work.
Fixes#7673