reactor: use correct open_flags when opening a file without DMA support
reactor: use correct open_flags when opening a file without DMA support

We go to the trouble of dropping O_DIRECT from open_flags, then don't use
open_flags at all.  This results in exclusive opens failing on tmpfs.

See scylladb/scylla#871.

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