gdth: Prevent negative offsets in ioctl CVE-2009-3080

commit 690e7448

 upstream.

A negative offset could be used to index before the event buffer and
lead to a security breach.

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>