FS-Cache: Provide a slab for cookie allocation

Provide a slab from which can be allocated the FS-Cache cookies that will be
presented to the netfs.

Also provide a slab constructor and a function to recursively discard a cookie
and its ancestor chain.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>
diff --git a/fs/fscache/Makefile b/fs/fscache/Makefile
index 556708b..f88ac17 100644
--- a/fs/fscache/Makefile
+++ b/fs/fscache/Makefile
@@ -4,6 +4,7 @@
 
 fscache-y := \
 	cache.o \
+	cookie.o \
 	fsdef.o \
 	main.o