commitlog: make new segments more xfs-friendly
commitlog: make new segments more xfs-friendly

xfs doesn't like writes beyond eof (exactly at eof is fine), and due
to continuation reordering, we sometimes do that.

Fix by pre-truncating the segment to its maximum size.

kjniemi/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla