cifs: fix memory leak in SMB2_read

Commit 088aaf17

 introduced a leak where
if SMB2_read() returned an error we would return without freeing the
request buffer.

Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>