Refer:
Linux Kernel struct udphdr data-structure ↗
sk_buff to udphdr APIs ↗
Definitions for the UDP module ↗
And here is the copy paste of struct udp data-structure (/include/uapi/linux/udp.h) from the Kernel-source version 4.13 for quick reference:
struct udphdr {
__be16 source;
__be16 dest;
__be16 len;
__sum16 check;
};