re: xen-blkback: move free persistent grants code

Hello Roger Pau Monne,

The patch 4d4f270f1880: "xen-blkback: move free persistent grants
code" from Nov 16, 2012, leads to the following warning:
drivers/block/xen-blkback/blkback.c:238 free_persistent_gnts()
	 warn: 'persistent_gnt' was already freed.

drivers/block/xen-blkback/blkback.c
   232                  pages[segs_to_unmap] = persistent_gnt->page;
   233                  rb_erase(&persistent_gnt->node, root);
   234                  kfree(persistent_gnt);
                        ^^^^^^^^^^^^^^^^^^^^
kfree();

   235                  num--;
   236  
   237                  if (++segs_to_unmap == BLKIF_MAX_SEGMENTS_PER_REQUEST ||
   238                          !rb_next(&persistent_gnt->node)) {
                                         ^^^^^^^^^^^^^^^^^^^^^
Dereferenced inside the call to rb_next().

   239                          ret = gnttab_unmap_refs(unmap, NULL, pages,
   240                                  segs_to_unmap);

regards,
dan carpenter