ieee1394: csr1212: proper refcounting

At least since nodemgr got rid of coarse global locking, accesses to
struct csr1212_keyval's reference counter should be atomic and coupled
with proper barriers.  Also, calls to csr1212_keep_keyval(kv) should
occur before kv is being used.

(We probably should convert refcnt to struct kref, but how to keep
csr1212_destroy_keyval's implementation non-recursively then?)

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>