LinuxKPI: skbuff updates

Various updates to skbuff for new/updated drivers and some housekeeping:

• update types and struct members, add new (stub) functions
• improve freeing of frags.
• fix an issue with sleeping during alloc for dev_alloc_skb().
• Adjust a KASSERT for skb_reserve() which apparently can be called multiple times if no data was put into the skb yet.
• move the sysctl from linux_8022.c (which may be in a different module) to linux_skbuff.c so in case we turn debugging on we do not run into unresolved symbols. Rename the sysctl variable to be less conflicting and update debugging macros along with that; also add IMPROVE().
• add DDB support to show an skbuff.
• adjust comments/whitespace.

No functional changes intended for iwlwifi.

Sponsored by: The FreeBSD Foundation (partially)
MFC after: 3 days