LinuxKPI: skbuff.h: fix -Warray-bounds warnings
Harmonize sk_buff_head and sk_buff further and fix -Warray-bounds
warnings reports by gcc. At the same time simplify some code by
re-using other functions or factoring some code out.
Sponsored by: The FreeBSD Foundation
(cherry picked from commit b2dcb848688e1e8346762531208dd7d468ff3854)