xfrm: Fix key lengths for rfc3686(ctr(aes))

Fix the min and max bit lengths for AES-CTR (RFC3686) keys.
The number of bits in key spec is the key length (128/256)
plus 32 bits of nonce.

This change takes care of the "Invalid key length" errors
reported by setkey when specifying 288 bit keys for aes-ctr.

Signed-off-by: Tushar Gohad <tgohad@mvista.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>