docs: filesystems: caching/backend-api.txt: convert it to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add it to filesystems/caching/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/5d0a61abaa87bfe913b9e2f321e74ef7af0f3dfc.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/caching/fscache.rst

@@ -187,7 +187,7 @@ The netfs API to FS-Cache can be found in:
 
 The cache backend API to FS-Cache can be found in:
 
-	Documentation/filesystems/caching/backend-api.txt
+	Documentation/filesystems/caching/backend-api.rst
 
 A description of the internal representations and object state machine can be
 found in:

Documentation/filesystems/caching/index.rst

@@ -8,6 +8,7 @@ Filesystem Caching
 
    fscache
    object
+   backend-api
    cachefiles
    netfs-api
    operations

fs/fscache/cache.c

@@ -172,7 +172,7 @@ no_preference:
  *
  * Initialise a record of a cache and fill in the name.
  *
- * See Documentation/filesystems/caching/backend-api.txt for a complete
+ * See Documentation/filesystems/caching/backend-api.rst for a complete
  * description.
  */
 void fscache_init_cache(struct fscache_cache *cache,
@@ -207,7 +207,7 @@ EXPORT_SYMBOL(fscache_init_cache);
  *
  * Add a cache to the system, making it available for netfs's to use.
  *
- * See Documentation/filesystems/caching/backend-api.txt for a complete
+ * See Documentation/filesystems/caching/backend-api.rst for a complete
  * description.
  */
 int fscache_add_cache(struct fscache_cache *cache,
@@ -307,7 +307,7 @@ EXPORT_SYMBOL(fscache_add_cache);
  * Note that an I/O error occurred in a cache and that it should no longer be
  * used for anything.  This also reports the error into the kernel log.
  *
- * See Documentation/filesystems/caching/backend-api.txt for a complete
+ * See Documentation/filesystems/caching/backend-api.rst for a complete
  * description.
  */
 void fscache_io_error(struct fscache_cache *cache)
@@ -355,7 +355,7 @@ static void fscache_withdraw_all_objects(struct fscache_cache *cache,
  * Withdraw a cache from service, unbinding all its cache objects from the
  * netfs cookies they're currently representing.
  *
- * See Documentation/filesystems/caching/backend-api.txt for a complete
+ * See Documentation/filesystems/caching/backend-api.rst for a complete
  * description.
  */
 void fscache_withdraw_cache(struct fscache_cache *cache)

fs/fscache/object.c

@@ -295,7 +295,7 @@ static void fscache_object_work_func(struct work_struct *work)
  *
  * Initialise a cache object description to its basic values.
  *
- * See Documentation/filesystems/caching/backend-api.txt for a complete
+ * See Documentation/filesystems/caching/backend-api.rst for a complete
  * description.
  */
 void fscache_object_init(struct fscache_object *object,

include/linux/fscache-cache.h

@@ -6,7 +6,7 @@
  *
  * NOTE!!! See:
  *
- *	Documentation/filesystems/caching/backend-api.txt
+ *	Documentation/filesystems/caching/backend-api.rst
  *
  * for a description of the cache backend interface declared here.
  */
@@ -454,7 +454,7 @@ static inline void fscache_object_lookup_error(struct fscache_object *object)
  * Set the maximum size an object is permitted to reach, implying the highest
  * byte that may be written.  Intended to be called by the attr_changed() op.
  *
- * See Documentation/filesystems/caching/backend-api.txt for a complete
+ * See Documentation/filesystems/caching/backend-api.rst for a complete
  * description.
  */
 static inline