mkfs: fix mkfs -r to properly allocate space

mkfs: fix mkfs -r to properly allocate space

mkfs -r wasn't creating chunks properly, making it very difficult to
allocate space for anything except tiny filesystems.

This changes it around to use more of the generic infrastructure, and
to do actual logical->physical block number translation.

It also allocates space to the files in smaller extents (max 1MB), which
keeps the allocator from trying to allocate an extent bigger than a
single chunk.

It doesn't quite support multi-device mkfs -r yet, but is much closer.

Signed-off-by: Chris Mason <chris.mason@fusionio.com>