io_tester: Set extent allocation size hint
io_tester: Set extent allocation size hint

The files are created with a bunch of writes, setting this
hint helps xfs to place the file in a more optimal way.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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