xen-blkfront: don't add indirect pages to list when !feature_persistent

xen-blkfront: don't add indirect pages to list when !feature_persistent

We should consider info->feature_persistent when adding indirect page to list
info->indirect_pages, else the BUG_ON() in blkif_free() would be triggered.

When we are using persistent grants the indirect_pages list
should always be empty because blkfront has pre-allocated enough
persistent pages to fill all requests on the ring.

CC: stable@vger.kernel.org
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit 7b0767502b5db11cb1f0daef2d01f6d71b1192dc)
Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>