FS-Cache: Object management state machine

Implement the cache object management state machine.

The following documentation is added to illuminate the working of this state
machine.  It will also be added as:

	Documentation/filesystems/caching/object.txt

	     ====================================================
	     IN-KERNEL CACHE OBJECT REPRESENTATION AND MANAGEMENT
	     ====================================================

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REPRESENTATION
==============

FS-Cache maintains an in-kernel representation of each object that a netfs is
currently interested in.  Such objects are represented by the fscache_cookie
struct and are referred to as cookies.

FS-Cache also maintains a separate in-kernel representation of the objects that
a cache backend is currently actively caching.  Such objects are represented by
the fscache_object struct.  The cache backends allocate these upon request, and
are expected to embed them in their own representations.  These are referred to
as objects.

There is a ...