support: Fix detecting hole support on >2KB-block filesystems

When detecting hole support, we write at 16MiB, and filesystems will
typically need two levels of data to record that. On filesystems with
8KB block, the two indirection blocks will require a total of 16KB
overhead, thus 32 512-byte sectors.

Spotted on GNU/Hurd with a 4KB blocks filesystem, but also happens on Linux
with 4KB or 8KB blocks filesystems.

* support/support_descriptor_supports_holes.c
(support_descriptor_supports_holes): Set block_headroom to 32.