xen/blkfront: always allocate grants first from per-queue persistent grants

This patch partially reverts 3df0e505 ("xen/blkfront: pseudo support for
multi hardware queues/rings"). The xen-blkfront queue/ring might hang due
to grants allocation failure in the situation when gnttab_free_head is
almost empty while many persistent grants are reserved for this queue/ring.

As persistent grants management was per-queue since 73716df7 ("xen/blkfront:
make persistent grants pool per-queue"), we should always allocate from
persistent grants first.

Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>