mem: fix typo in determining an address of parent vma

Look at this hunk from 7659c995:
-    paddr = decode_pointer(vma_premmaped_start(&p->vma));
+    paddr = decode_pointer(vma->premmaped_addr);

Obviously we want to use p->premmaped_addr instead of
vma->premmaped_addr.

Fixes: 7659c995 ("vm: don't overwrite vma->shmid for private mappings")
Reported-by: Filipe Brandenburger <filbranden@google.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -260,7 +260,7 @@ static int map_private_vma(pid_t pid, struct vma_area *vma, void **tgt_addr,
 
 		pr_info("COW 0x%016"PRIx64"-0x%016"PRIx64" 0x%016"PRIx64" vma\n",
 			vma->e->start, vma->e->end, vma->e->pgoff);
-		paddr = decode_pointer(vma->premmaped_addr);
+		paddr = decode_pointer(p->premmaped_addr);
 	}
 
 	*pvma = p;