bcachefs: Validate number of counters for accounting keys

bcachefs: Validate number of counters for accounting keys

We weren't checking that accounting keys have the expected number of
accounters. Originally we probably wanted to be flexible on this, but it
doesn't look like that will be required - accounting is extended by
adding new counter types, not more counters to an existing type.

This means we can drop a BUG_ON() that popped once in automated testing,
and the new validation will make that bug easier to track down.

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