mkfs+mount failure of small fs on ppc64

[Eric Sandeen <sandeen@redhat.com>]

on ppc64, 4.7-rc kernel, git btrfs-progs, v4.7.2:

# truncate --size=500m testfile
# ./mkfs.btrfs testfile
# mkdir -p mnt
# mount -o loop testfile mnt

btrfs-progs v4.7.2
See http://btrfs.wiki.kernel.org for more information.

Label:              (null)
UUID:               c531b759-a491-4c9f-a954-4787cea9106d
Node size:          65536
Sector size:        65536
Filesystem size:    500.00MiB
Block group profiles:
  Data:             single            8.00MiB
  Metadata:         DUP              32.00MiB
  System:           DUP               8.00MiB
SSD detected:       no
Incompat features:  extref, skinny-metadata
Number of devices:  1
Devices:
   ID        SIZE  PATH
    1   500.00MiB  testfile


# dmesg -c
[   61.210287] loop: module loaded
[   61.247105] BTRFS: device fsid a8d79cd0-977f-4b93-8410-246dc08b3683 devid 1 transid 5 /dev/loop0
[   61.247391] BTRFS info (device loop0): disk space caching is enabled
[   61.247397] BTRFS info (device loop0): has skinny extents
[   61.270492] BTRFS info (device loop0): creating UUID tree
[   61.312149] BTRFS warning (device loop0): failed to create the UUID tree: -28
[   61.483028] BTRFS: open_ctree failed

2nd mount works:

# mount -o loop testfile mnt
# dmesg -c
[   87.504564] BTRFS info (device loop0): disk space caching is enabled
[   87.504579] BTRFS info (device loop0): has skinny extents
[   87.506979] BTRFS info (device loop0): creating UUID tree

Any ideas?  This seems to have regressed since 3.9.1, but there are a couple
other mkfs breakages in between, and my bisect was not fruitful.

Thanks,
-Eric