fstream: remove stream-based preallocation
fstream: remove stream-based preallocation

Better imlemented in the file system itself.

sjperkins/seastar,kjniemi/seastar,dreamsxin/seastar,kjniemi/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,sjperkins/seastar,avikivity/seastar,cloudius-systems/seastar,dreamsxin/seastar,scylladb/seastar,dreamsxin/seastar,scylladb/scylla-seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/scylla-seastar,cloudius-systems/seastar,scylladb/scylla-seastar,avikivity/seastar,scylladb/seastar,sjperkins/seastar,kjniemi/seastar