LinuxKPI: skbuff: add synchronization primitives and missing bits

Make a pass over skbuff.h:

• implement some missing bits,
• sprinkle some const,
• add locking and read/write_once calls as needed to provide synchronization as expected by Linux,
• fix some typos,
• remove return from void functions,
• adjust tracing macros.

Sponsored by: The FreeBSD Foundation
PR: 283903 (rtw88 skb leak)
Tested by: Guillaume Outters (guillaume-freebsd outters.eu)
Tested by: oleg.nauman gmail.com
Differential Revision: https://reviews.freebsd.org/D49101

(cherry picked from commit 2ab4a41956159e7c974979693cb0b13cf552128e)