CRED: Fix kernel panic upon security_file_alloc() failure.

In get_empty_filp() since 2.6.29, file_free(f) is called with f->f_cred == NULL
when security_file_alloc() returned an error.  As a result, kernel will panic()
due to put_cred(NULL) call within RCU callback.

Fix this bug by assigning f->f_cred before calling security_file_alloc().
Signed-off-by: NTetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/file_table.c

@@ -125,13 +125,13 @@ struct file *get_empty_filp(void)
 		goto fail;
 
 	percpu_counter_inc(&nr_files);
+	f->f_cred = get_cred(cred);
 	if (security_file_alloc(f))
 		goto fail_sec;
 
 	INIT_LIST_HEAD(&f->f_u.fu_list);
 	atomic_long_set(&f->f_count, 1);
 	rwlock_init(&f->f_owner.lock);
-	f->f_cred = get_cred(cred);
 	spin_lock_init(&f->f_lock);
 	eventpoll_init_file(f);
 	/* f->f_version: 0 */