KEYS: Fix skcipher IV clobbering

The IV must not be modified by the skcipher operation so we need
to duplicate it.

Fixes: c3917fd9

 ("KEYS: Use skcipher")
Cc: stable@vger.kernel.org
Reported-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>