gru: check for valid vma

Fix bug caused by failure to allocate a GRU gts structure.  The old code
failed to handle the case where the vma was invalid.
Signed-off-by: NJack Steiner <steiner@sgi.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/misc/sgi-gru/grufault.c

@@ -90,19 +90,22 @@ static struct gru_thread_state *gru_alloc_locked_gts(unsigned long vaddr)
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma;
-	struct gru_thread_state *gts = NULL;
+	struct gru_thread_state *gts = ERR_PTR(-EINVAL);
 
 	down_write(&mm->mmap_sem);
 	vma = gru_find_vma(vaddr);
-	if (vma)
-		gts = gru_alloc_thread_state(vma, TSID(vaddr, vma));
-	if (!IS_ERR(gts)) {
-		mutex_lock(&gts->ts_ctxlock);
-		downgrade_write(&mm->mmap_sem);
-	} else {
-		up_write(&mm->mmap_sem);
-	}
+	if (!vma)
+		goto err;
+
+	gts = gru_alloc_thread_state(vma, TSID(vaddr, vma));
+	if (IS_ERR(gts))
+		goto err;
+	mutex_lock(&gts->ts_ctxlock);
+	downgrade_write(&mm->mmap_sem);
+	return gts;
 
+err:
+	up_write(&mm->mmap_sem);
 	return gts;
 }