exfat: fdatasync flag should be same like generic_write_sync()

Test: androbench by default setting, use 64GB sdcard.
 the random write speed:
	without this patch 3.5MB/s
	with this patch 7MB/s

After patch "11a347fb", the random write speed decreased significantly.
the .write_iter() interface had been modified, and check the differences
with generic_file_write_iter(), when calling generic_write_sync() and
exfat_file_write_iter() to call vfs_fsync_range(), the fdatasync flag is
wrong, and make not use the fdatasync mode, and make random write speed
decreased. So use generic_write_sync() instead of vfs_fsync_range().

Fixes: 11a347fb ("exfat: change to get file size from DataLength")
Signed-off-by: Zhengxu Zhang <zhengxu.zhang@unisoc.com>
Acked-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>