f2fs: remove redundant check in f2fs_file_write_iter()

We have already checked flag IOCB_DIRECT in the sanity
check of flag IOCB_NOWAIT, so don't have to check it
again here.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>