lib: fix test_hmm.c reference after free

Coccinelle scripts report the following errors:

  lib/test_hmm.c:523:20-26: ERROR: reference preceded by free on line 521
  lib/test_hmm.c:524:21-27: ERROR: reference preceded by free on line 521
  lib/test_hmm.c:523:28-35: ERROR: devmem is NULL but dereferenced.
  lib/test_hmm.c:524:29-36: ERROR: devmem is NULL but dereferenced.

Fix these by using the local variable 'res' instead of devmem.

Link: http://lkml.kernel.org/r/c845c158-9c65-9665-0d0b-00342846dd07@infradead.orgSigned-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/test_hmm.c

@@ -520,8 +520,7 @@ static bool dmirror_allocate_chunk(struct dmirror_device *mdevice,
 err_free:
 	kfree(devmem);
 err_release:
-	release_mem_region(devmem->pagemap.res.start,
-			   resource_size(&devmem->pagemap.res));
+	release_mem_region(res->start, resource_size(res));
 err:
 	mutex_unlock(&mdevice->devmem_lock);
 	return false;