LinuxKPI: skbuff: add a misplaced socket operation to skbuff.h for now

This likely belongs in socket code which we do not have in LinuxKPI.
Needed by a wirless driver at v6.17.

Approved by: re (cperciva)

(cherry picked from commit 88dbf83345feefa7181bd6df47786e1a8d1d304d)
(cherry picked from commit 3ad05fa24a240011a8252140ff6431cb0b7879d9)