When disk space accounting was changed to be tracked by replicas entry,
the ioctl interface was never update: this patch finally does that.

Aditionally, the BCH_IOCTL_USAGE ioctl is now broken out into separate
ioctls for filesystem and device usage.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>