xen-blkfront: switch from llist to list

xen-blkfront: switch from llist to list

The git commit 7c6ef2b69a43a3d98de84cf3403bee45f87bd29f
(xen-blkfront: drop the use of llist_for_each_entry_safe)

was a stop-gate to fix a GCC4.1 bug. The appropiate way
is to actually use an list instead of using an llist.

As such this patch replaces the usage of llist with an
list.

Since we always manipulate the list while holding the io_lock, there's
no need for additional locking (llist used previously is safe to use
concurrently without additional locking).

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
CC: stable@vger.kernel.org
[v1: Redid the git commit description]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>