bcachefs: bch_alloc->stripe_sectors

Add a separate counter to bch_alloc_v4 for amount of striped data; this
lets us separately track striped and unstriped data in a bucket, which
lets us see when erasure coding has failed to update extents with stripe
pointers, and also find buckets to continue updating if we crash mid way
through creating a new stripe.

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