We need to make sure that the device is not RO or that
the request is not past the number of sectors we want to
issue the DISCARD operation for.

This fixes CVE-2013-2140.

Cc: stable@vger.kernel.org
Acked-by: NJan Beulich <JBeulich@suse.com>
Acked-by: NIan Campbell <Ian.Campbell@citrix.com>
[v1: Made it pr_warn instead of pr_debug]
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>