bcachefs: BCH_IOCTL_QUERY_ACCOUNTING

bcachefs: BCH_IOCTL_QUERY_ACCOUNTING

Add a new ioctl that can return the new accounting counter types; it
takes as input a bitmask of accounting types to return.

This will be used for returning e.g. compression accounting and
rebalance_work accounting.

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