crypto: rng - Make crypto_stdrng_get_bytes() use normal RNG in non-FIPS mode

crypto: rng - Make crypto_stdrng_get_bytes() use normal RNG in non-FIPS mode

"stdrng" is needed only in "FIPS mode".  Therefore, make
crypto_stdrng_get_bytes() delegate to either the normal Linux RNG or to
"stdrng", depending on the current mode.

This will eliminate the need to built the SP800-90A DRBG and its
dependencies into CRYPTO_FIPS=n kernels.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>