keys: fix trusted/encrypted keys sparse rcu_assign_pointer messages

Define rcu_assign_keypointer(), which uses the key payload.rcudata instead
of payload.data, to resolve the CONFIG_SPARSE_RCU_POINTER message:
"incompatible types in comparison expression (different address spaces)"

Replace the rcu_assign_pointer() calls in encrypted/trusted keys with
rcu_assign_keypointer().

Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>