Add an RCU payload dereference macro as this seems to be a common piece of code
amongst key types that use RCU referenced payloads.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NMimi Zohar <zohar@us.ibm.com>
Signed-off-by: NJames Morris <jmorris@namei.org>