crypto: testmgr - Allow ecb(cipher_null) in FIPS mode

The cipher_null is not a real cipher, FIPS mode should not restrict its use.

It is used for several tests (for example in cryptsetup testsuite) and also
temporarily for reencryption of not yet encrypted device in cryptsetup-reencrypt tool.

Problem is easily reproducible with
  cryptsetup benchmark -c null

Signed-off-by: Milan Broz <gmazyland@gmail.com>
Acked-by: Stephan Müller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>