keys: fix trusted/encrypted keys sparse rcu_assign_pointer messages

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 <[email protected]>
Signed-off-by: David Howells <[email protected]>
Signed-off-by: James Morris <[email protected]>