LinuxKPI: add type for __wsum

Seems this is the correct type for the csum field in an sk_buff.
Add it as uint32_t bitwise.

Sponsored by; The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49831

(cherry picked from commit c925e0a77606d07e4d93204a1d35530176c4b344)