fscache: Count data storage objects in a cache

Count the data storage objects that are currently allocated in a cache.
This is used to pin certain cache structures until cache withdrawal is
complete.

Three helpers are provided to manage and make use of the count:

 (1) void fscache_count_object(struct fscache_cache *cache);

     This should be called by the cache backend to note that an object has
     been allocated and attached to the cache.

 (2) void fscache_uncount_object(struct fscache_cache *cache);

     This should be called by the backend to note that an object has been
     destroyed.  This sends a wakeup event that allows cache withdrawal to
     proceed if it was waiting for that object.

 (3) void fscache_wait_for_objects(struct fscache_cache *cache);

     This can be used by the backend to wait for all outstanding cache
     object to be destroyed.

Each cache's counter is displayed as part of /proc/fs/fscache/caches.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819608594.215744.1812706538117388252.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906911646.143852.168184059935530127.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967111846.1823006.9868154941573671255.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021516219.640689.4934796654308958158.stgit@warthog.procyon.org.uk/ # v4

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