f2fs: remove duplicate code in f2fs_file_write_iter

We will do the same check in generic_write_checks.
if (iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT)
        return -EINVAL;
just remove the same check in f2fs_file_write_iter.

Signed-off-by: Lihong Kou <koulihong@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>