bcachefs: fix bch2_dev_usage_full_read_fast()

One reference to bch_dev_usage wasn't updated, which meant we weren't
reading the full bch_dev_usage_full - oops.

Fixes: 955ba7b5 ("bcachefs: bch_dev_usage_full")
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>