ext4: fix use-after-free race in ext4_remount()'s error path

ext4: fix use-after-free race in ext4_remount()'s error path

commit 33458eaba4dfe778a426df6a19b7aad2ff9f7eec upstream.

It's possible for ext4_show_quota_options() to try reading
s_qf_names[i] while it is being modified by ext4_remount() --- most
notably, in ext4_remount's error path when the original values of the
quota file name gets restored.

Reported-by: syzbot+a2872d6feea6918008a9@syzkaller.appspotmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org # 3.2+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>