crypto: xts - Only add ecb if it is not already there

Only add ecb to the cipher name if it isn't already ecb.

Also use memcmp instead of strncmp since these strings are all
stored in an array of length CRYPTO_MAX_ALG_NAME.

Fixes: f1c131b4 ("crypto: xts - Convert to skcipher")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>