fstream: fix condition when buffer size can be increased
fstream: fix condition when buffer size can be increased

avikivity/seastar,dreamsxin/seastar,scylladb/seastar,avikivity/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,cloudius-systems/seastar,avikivity/seastar,dreamsxin/seastar,syuu1228/seastar,dreamsxin/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar