f2fs: Change the type of f2fs_flush_inline_data() to void

The return value of f2fs_flush_inline_data() is not used,
so delete it.

Signed-off-by: Jia Yang <jiayang5@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>