fs/fscache/object-list.c: use __seq_open_private()

Reduce boilerplate code by using __seq_open_private() instead of seq_open()
in fscache_objlist_open().

Signed-off-by: Rob Jones <rob.jones@codethink.co.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>