dm kcopyd: prepare for callback race fix

Use a variable in segment_complete() to point to the dm_kcopyd_client
struct and only release job->pages in run_complete_job() if any are
defined.  These changes are needed by the next patch.

Cc: stable@kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-kcopyd.c

@@ -297,7 +297,8 @@ static int run_complete_job(struct kcopyd_job *job)
 	dm_kcopyd_notify_fn fn = job->fn;
 	struct dm_kcopyd_client *kc = job->kc;
 
-	kcopyd_put_pages(kc, job->pages);
+	if (job->pages)
+		kcopyd_put_pages(kc, job->pages);
 	mempool_free(job, kc->job_pool);
 	fn(read_err, write_err, context);
 
@@ -461,6 +462,7 @@ static void segment_complete(int read_err, unsigned long write_err,
 	sector_t progress = 0;
 	sector_t count = 0;
 	struct kcopyd_job *job = (struct kcopyd_job *) context;
+	struct dm_kcopyd_client *kc = job->kc;
 
 	mutex_lock(&job->lock);
 
@@ -490,7 +492,7 @@ static void segment_complete(int read_err, unsigned long write_err,
 
 	if (count) {
 		int i;
-		struct kcopyd_job *sub_job = mempool_alloc(job->kc->job_pool,
+		struct kcopyd_job *sub_job = mempool_alloc(kc->job_pool,
 							   GFP_NOIO);
 
 		*sub_job = *job;