Previously, bucket fragmentation was considered to be bucket size -
total amount of live data, both dirty and cached.

This meant that if a bucket was full but only a small amount of data in
it was dirty - the rest cached, we'd get stuck: copygc wouldn't move the
dirty data out of the bucket and the allocator wouldn't be able to
invalidate and drop the cached data.

This changes fragmentation to exclude cached data, so that copygc will
evacuate these buckets and copygc/the allocator will always be able to
make forward progress.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>