bcachefs: Convert disk accounting BUG_ON() to WARN_ON()

We had a bug where disk accounting keys didn't always have their version
field set in journal replay; change the BUG_ON() to a WARN(), and
exclude this case since it's now checked for elsewhere (in the bkey
validate function).

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