docs: filesystems: caching/backend-api.rst: correct two relinquish APIs use

1. cache backend is using fscache_relinquish_cache() rather than
   fscache_relinquish_cookie() to reset the cache cookie.

2. No fscache_cache_relinquish() helper currently, it should be
   fscache_relinquish_cache().

Signed-off-by: Yue Hu <huyue2@coolpad.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://listman.redhat.com/archives/linux-cachefs/2022-April/006703.html # v1
Link: https://listman.redhat.com/archives/linux-cachefs/2022-April/006704.html # v2

Author: taigerhu

Documentation/filesystems/caching/backend-api.rst

@@ -73,7 +73,7 @@ busy.
 If successful, the cache backend can then start setting up the cache.  In the
 event that the initialisation fails, the cache backend should call::
 
-	void fscache_relinquish_cookie(struct fscache_cache *cache);
+	void fscache_relinquish_cache(struct fscache_cache *cache);
 
 to reset and discard the cookie.
 
@@ -125,7 +125,7 @@ outstanding accesses on the volume to complete before returning.
 When the the cache is completely withdrawn, fscache should be notified by
 calling::
 
-	void fscache_cache_relinquish(struct fscache_cache *cache);
+	void fscache_relinquish_cache(struct fscache_cache *cache);
 
 to clear fields in the cookie and discard the caller's ref on it.