Commit a46b5367 ("xen/blkfront: cleanup stale persistent grants")
added support for purging persistent grants when they are not in use. As
part of the purge, the grants were removed from the grant buffer, This
eventually causes the buffer to become empty, with BUG_ON triggered in
get_free_grant(). This can be observed even on an idle system, within
20-30 minutes.

We should keep the grants in the buffer when purging, and only free the
grant ref.

Fixes: a46b5367 ("xen/blkfront: cleanup stale persistent grants")
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>