At function exit, do not leave the expanded key in the rk struct
which got allocated on the stack.

Signed-off-by: Torsten Duwe <duwe@suse.de>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>