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
MFC after: 3 days
PR: 283903 (rtw88 skb leak)
Tested by: Guillaume Outters (guillaume-freebsd outters.eu)
Differential Revision: https://reviews.freebsd.org/D49101