Fix a sockbuf race. Currently the m_free*() path can block, due to

Fix a sockbuf race.  Currently the m_free*() path can block, due to
objcache_put() blocking when it must access the global depot.  This breaks
the critical section *AND the BGL during a time when the sockbuf state is
inconsistent.  Another process accessing the same sockbuf would then
corrupt it.  Since depot access is fairly rare, this bug typically required
a number of hours to reproduce.

Delay the actual freeing of mbufs until after the sockbuf state has been
updated.  Encapsulate common operations in a procedure, and add additional
assertions.  NULL out sb_lastrecord when it becomes invalid, and add a
considerable amount of debugging code.

SOCKBUF_DEBUG has been added.  Note that this is a VERY EXPENSIVE
kernel compile option which should only be used when specifically debugging
the networking subsystem.

This is a stabilization patch is rather hackish.  A better cleanup will
occur once we are sure we've fixed all the bugs.

sbcheck provided by: Jeffrey Hsu
Reported-by: David Rhodus, Peter Avalos, YONETANI Tomokazu,
	     Tomaz Borstnar, and numerous other people.