Replace all fsync() with fdatasync() when possible (e.g. On Linux)

InnoDB doesn't care about file timestamps. So, to achieve a better
performance it makes sense to use fdatasync() everywhere.

file_io::flush(): renamed from flush_data_only()

os_file_flush_data(): removed

os_file_sync_posix(): renamed from os_file_fsync_posix(). Now it uses
fdatasync() when it's available.