f2fs: remove unnecessary IS_SWAPFILE check

Now swapfile in f2fs directly submit IO to blockdev according to
swapfile extents reported by f2fs when swapon, therefore there is
no need to check IS_SWAPFILE when exec filesystem operation.

Signed-off-by: Huang Jianan <huangjianan@oppo.com>
Signed-off-by: Guo Weichao <guoweichao@oppo.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>