FS-Cache: Start processing an object's operations on that object's death

Start processing an object's operations when that object moves into the DYING
state as the object cannot be destroyed until all its outstanding operations
have completed.

Furthermore, make sure that read and allocation operations handle being woken
up on a dead object.  Such events are recorded in the Allocs.abt and
Retrvls.abt statistics as viewable through /proc/fs/fscache/stats.

The code for waiting for object activation for the read and allocation
operations is also extracted into its own function as it is much the same in
all cases, differing only in the stats incremented.

Signed-off-by: David Howells <dhowells@redhat.com>

Documentation/filesystems/caching/fscache.txt

@@ -253,6 +253,7 @@ proc files.
 		int=N	Number of alloc reqs aborted -ERESTARTSYS
 		ops=N	Number of alloc reqs submitted
 		owt=N	Number of alloc reqs waited for CPU time
+		abt=N	Number of alloc reqs aborted due to object death
 	Retrvls	n=N	Number of retrieval (read) requests seen
 		ok=N	Number of successful retr reqs
 		wt=N	Number of retr reqs that waited on lookup completion
@@ -262,6 +263,7 @@ proc files.
 		oom=N	Number of retr reqs failed -ENOMEM
 		ops=N	Number of retr reqs submitted
 		owt=N	Number of retr reqs waited for CPU time
+		abt=N	Number of retr reqs aborted due to object death
 	Stores	n=N	Number of storage (write) requests seen
 		ok=N	Number of successful store reqs
 		agn=N	Number of store reqs on a page already pending storage