bcachefs: BCH_IOCTL_DEV_USAGE_V2

BCH_IOCTL_DEV_USAGE mistakenly put the per-data-type array in struct
bch_ioctl_dev_usage; since ioctl numbers encode the size of the arg,
that means adding new data types breaks the ioctl.

This adds a new version that includes the number of data types as a
parameter: the old version is fixed at 10 so as to not break when adding
new types.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>