Since commit d05d7f40 ("Merge branch 'for-4.8/core' of
git://git.kernel.dk/linux-block") and 3fc9d690 ("Merge branch
'for-4.8/drivers' of git://git.kernel.dk/linux-block"), blkfront_resume()
has been using an index for iterating ring_info to check request when
iterating blk_shadow in an inner loop. This seems to have been
accidentally introduced during the massive rewrite of the block layer
macros in the commits.

This may cause crash like this:

[11798.057074] BUG: unable to handle kernel NULL pointer dereference at 0000000000000048
[11798.058832] IP: [<ffffffff814411fa>] blkfront_resume+0x10a/0x610
....
[11798.061063] Call Trace:
[11798.061063]  [<ffffffff8139ce93>] xenbus_dev_resume+0x53/0x140
[11798.061063]  [<ffffffff8139ce40>] ? xenbus_dev_probe+0x150/0x150
[11798.061063]  [<ffffffff813f359e>] dpm_run_callback+0x3e/0x110
[11798.061063]  [<ffffffff813f3a08>] device_resume+0x88/0x190
[11798.061063]  [<ffffffff813f4cc0>] dpm_resume+0x100/0x2d0
[11798.061063]  [<ffffffff813f5221>] dpm_resume_end+0x11/0x20
[11798.061063]  [<ffffffff813950a8>] do_suspend+0xe8/0x1a0
[11798.061063]  [<ffffffff813954bd>] shutdown_handler+0xfd/0x130
[11798.061063]  [<ffffffff8139aba0>] ? split+0x110/0x110
[11798.061063]  [<ffffffff8139ac26>] xenwatch_thread+0x86/0x120
[11798.061063]  [<ffffffff810b4570>] ? prepare_to_wait_event+0x110/0x110
[11798.061063]  [<ffffffff8108fe57>] kthread+0xd7/0xf0
[11798.061063]  [<ffffffff811da811>] ? kfree+0x121/0x170
[11798.061063]  [<ffffffff8108fd80>] ? kthread_park+0x60/0x60
[11798.061063]  [<ffffffff810863b0>] ?  call_usermodehelper_exec_work+0xb0/0xb0
[11798.061063]  [<ffffffff810864ea>] ?  call_usermodehelper_exec_async+0x13a/0x140
[11798.061063]  [<ffffffff81534a45>] ret_from_fork+0x25/0x30

Use the right index in the inner loop.

Fixes: d05d7f40 ("Merge branch 'for-4.8/core' of git://git.kernel.dk/linux-block")
Fixes: 3fc9d690 ("Merge branch 'for-4.8/drivers' of git://git.kernel.dk/linux-block")
Signed-off-by: Munehisa Kamata <kamatam@amazon.com>
Reviewed-by: Thomas Friebel <friebelt@amazon.de>
Reviewed-by: Eduardo Valentin <eduval@amazon.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Roger Pau Monne <roger.pau@citrix.com>
Cc: xen-devel@lists.xenproject.org
Cc: stable@vger.kernel.org
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>