COW header is zeroed when FS doesn't support fallocate

[crawfxrd]

This happens on:

• CentOS 6 (2.6.32)
• CentOS 7 (3.10)
• Ubuntu 16.04 (4.4)

This does not happen on:

• openSUSE 15 (4.12)
• Fedora 27 (4.18)

When using ext2, ext3, FAT32, or NTFS, the COW header is all zeros.
When using ext4 or XFS, the COW header has data.

fallocate -l 1G disk.img
losetup /dev/loop0 disk.img
mkfs.vfat -F32 /dev/loop0
mount /dev/loop0 /mnt
sync
dbdctl setup-snapshot /dev/loop0 /mnt/cow.snap 0

[root@segv-c7 ~]# xxd -l 64 /mnt/cow.snap
0000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................