Support seeking also from the end of file on S3 storage

The PR https://github.com/nextcloud/server/pull/20033

 added support
for `fseek` for  the S3 storage backend. However, the seek mode SEEK_END
was left out that time. This PR fills this gap.

Signed-off-by: Pauli Järvinen <pauli.jarvinen@gmail.com>