Enlarging w/ xfs_growfs: XFS_IOC_FSGROWFSDATA xfsctl failed: Inappropriate ioctl for device

[Nick Bowler <nbowler@draconx.ca>]

Hello,

I'm a bit new to using XFS and I ran into some errors trying to enlarge
a filesystem.  This setup uses dmcrypt on top of md raid and I just
reshaped the array to add additional storage.  The underlying block
device reflects the new size, but the filesystem hasn't been enlarged
yet:

  # blockdev --report /dev/mapper/data             
  RO    RA   SSZ   BSZ   StartSec            Size   Device
  rw  4096   512  4096          0  20001386921984   /dev/mapper/data

  # findmnt /dev/mapper/data
  TARGET    SOURCE           FSTYPE OPTIONS
  /mnt/data /dev/mapper/data xfs    rw,relatime,attr2,inode64,sunit=1024,swidth=2048,noquota

  # df -h /mnt/data
  Filesystem        Size  Used Avail Use% Mounted on
  /dev/mapper/data  9.1T  8.5T  649G  94% /mnt/data

So I read the manpage and it seems all I should need to do is run
xfs_growfs on the mounted filesystem but...

  # xfs_growfs /mnt/data
  meta-data=/dev/mapper/data       isize=512    agcount=32, agsize=76299136 blks
           =                       sectsz=4096  attr=2, projid32bit=1
           =                       crc=1        finobt=1, sparse=1, rmapbt=0
           =                       reflink=0
  data     =                       bsize=4096   blocks=2441572352, imaxpct=5
           =                       sunit=128    swidth=256 blks
  naming   =version 2              bsize=4096   ascii-ci=0, ftype=1
  log      =internal log           bsize=4096   blocks=521728, version=2
           =                       sectsz=4096  sunit=1 blks, lazy-count=1
  realtime =none                   extsz=4096   blocks=0, rtextents=0
  xfs_growfs: XFS_IOC_FSGROWFSDATA xfsctl failed: Inappropriate ioctl for device
  xfs_growfs: XFS_IOC_FSGEOMETRY xfsctl failed: Inappropriate ioctl for device

... and the filesystem is not enlarged.  Looking at strace output, the
failing ioctls seem to be:

  openat(AT_FDCWD, "/mnt/data", O_RDONLY) = 3
  [...]
  ioctl(3, _IOC(_IOC_WRITE, 0x58, 0x6e, 0x10), 0xffcc9a80) = -1 ENOTTY (Inappropriate ioctl for device)
  [...]
  ioctl(3, _IOC(_IOC_READ, 0x58, 0x64, 0x70), 0xffcc9ba0) = -1 ENOTTY (Inappropriate ioctl for device)

Kernel version is 4.14.82 with xfsprogs 4.17.0, although I tried also
with xfsprogs 4.19.0 and received the same errors.

Am I missing something obvious here?  What further steps should I take
to help solve this?

Thanks,
  Nick