CacheFiles: cachefiles_write_page() shouldn't indicate error twice

Prevent cachefiles_write_page() from issuing an error twice, instead forgoing the callback if it can return the error directly.

Previously, the netfs was being informed of the error twice, once by the error being passed to the netfs callback, and a second time when fscache_write_page() returned to the netfs.

In NFS's case, this meant it attempted to do error handling twice, including clearing PG_fs_misc twice...

---

 fs/cachefiles/cf-interface.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

index 6be3d98..4c0eb31 100644
@@ -1247,9 +1247,11 @@ #else
 						"write page to backing file"
 						" failed");
 		ret = -ENOBUFS;
+	} else {
+		/* only invoke the callback if successful, we return the error
+		 * directly otherwise */
+		end_io_func(page, context, ret);
 	}
-
-	end_io_func(page, context, ret);
 #endif
 
 	_leave(" = %d", ret);