reactor: switch to getdents64 for directory reads
reactor: switch to getdents64 for directory reads

Some architectures (ARMv8) don't have the old syscall. Since
getdents64 was introduced in 2.4, it should be available
everywhere.

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