Currently, we don't use bucket data type for tracking whether buckets
are part of a stripe; parity buckets are BCH_DATA_parity, but data
buckets in a stripe are BCH_DATA_user. There's a separate counter,
buckets_ec, outside the BCH_DATA_TYPES system for tracking number of
buckets on a device that are part of a stripe.

The trouble with this approach is that it's too coarse grained, and we
need better information on fragmentation for debugging copygc.

With this patch, data buckets in a stripe are now tracked as
BCH_DATA_stripe buckets.

This doesn't yet differentiate between erasure coded and non-erasure
coded data in a stripe bucket, nor do we yet track empty data buckets in
stripes.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>