I have a 10 x 2TB disk mdadm RAID5 volume. I added 4 more 2 TB disks, grew the array and tried to resize the partition.
I ran into this:

resize2fs: /dev/md8: The combination of flex_bg and
	!resize_inode features is not supported by resize2fs.

A little googling shows this:

which leads to this:

Is there really no way to resize this filesystem?