LinuxKPI: skbuff updates

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