bcachefs: bucket_data_type_mismatch()

We're working on potentially unifying bch2_check_bucket_ref() and
bch2_check_fix_ptrs() - or at least eliminating gratuitious differences.

Most immediately, there's a bunch of cleanups to be done regarding
BCH_DATA_stripe.

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