FS-Cache: Add and document asynchronous operation handling

Add and document asynchronous operation handling for use by FS-Cache's data
storage and retrieval routines.

The following documentation is added to:

	Documentation/filesystems/caching/operations.txt

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		       ASYNCHRONOUS OPERATIONS HANDLING
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========
OVERVIEW
========

FS-Cache has an asynchronous operations handling facility that it uses for its
data storage and retrieval routines.  Its operations are represented by
fscache_operation structs, though these are usually embedded into some other
structure.

This facility is available to and expected to be be used by the cache backends,
and FS-Cache will create operations and pass them off to the appropriate cache
backend for completion.

To make use of this facility, <linux/fscache-cache.h> should be #included.

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OPERATION RECORD INITIALISATION
===============================

An oper...