f2fs: remove redundant check of page type when submit bio

This patch removes redundant check of page type when submit bio to
make the logic more clear.

Signed-off-by: Tiezhu Yang <kernelpatch@126.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>