IB/mthca: Fix gup usage in mthca_map_user_db()

get_user_pages() must be called with mmap_sem held, currently
it is not. In fact it is called under the user db_table->mutex.
To fix this we can convert gup to use the fast alternative,
and safely avoid taking mmap_sem, if possible. Furthermore
this is safe wrt to the mutex as other callers that take the
lock (unmap and alloc_db) are not called under mmap_sem
(hence possible deadlock).

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/hw/mthca/mthca_memfree.c

@@ -472,7 +472,7 @@ int mthca_map_user_db(struct mthca_dev *dev, struct mthca_uar *uar,
 		goto out;
 	}
 
-	ret = get_user_pages(uaddr & PAGE_MASK, 1, FOLL_WRITE, pages, NULL);
+	ret = get_user_pages_fast(uaddr & PAGE_MASK, 1, FOLL_WRITE, pages);
 	if (ret < 0)
 		goto out;