ext4: Make Q_GETNEXTQUOTA work for quota in hidden inodes

We forgot to set .get_nextdqblk operation in quotactl_ops structure used
by ext4 when quota is using hidden inode thus the operation was not
really supported. Fix the omission.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Jan Kara <jack@suse.cz>