xfs: prevent rt growfs when quota is enabled

Quotas aren't (yet) supported with realtime, so we shouldn't allow
userspace to set up a realtime section when quotas are enabled, even if
they attached one via mount options.  IOWS, you shouldn't be able to do:

# mkfs.xfs -f /dev/sda
# mount /dev/sda /mnt -o rtdev=/dev/sdb,usrquota
# xfs_growfs -r /mnt

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>