f2fs: code cleanup by removing unnecessary check

f2fs_seek_block() is only used for regular file,
so don't have to check inline dentry in it.

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