FS-Cache: Add netfs registration

Add functions to register and unregister a network filesystem or other client
of the FS-Cache service.  This allocates and releases the cookie representing
the top-level index for a netfs, and makes it available to the netfs.

If the FS-Cache facility is disabled, then the calls are optimised away at
compile time.

Note that whilst this patch may appear to work with FS-Cache enabled and a
netfs attempting to use it, it will leak the cookie it allocates for the netfs
as fscache_relinquish_cookie() is implemented in a later patch.  This will
cause the slab code to emit a warning when the module is removed.

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 f88ac17..ecf6946 100644
--- a/fs/fscache/Makefile
+++ b/fs/fscache/Makefile
@@ -6,7 +6,8 @@
 	cache.o \
 	cookie.o \
 	fsdef.o \
-	main.o
+	main.o \
+	netfs.o
 
 fscache-$(CONFIG_PROC_FS) += proc.o
 fscache-$(CONFIG_FSCACHE_STATS) += stats.o