open_flags: Add O_DSYNC value
open_flags: Add O_DSYNC value

Enabling O_DSYNC mode for files.
Message-Id: <0482279d316764c1b07bba8ce681530ec9461594@scylladb.com>

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