crypto: api - Add crypto_stack_request_init and initialise flags fully

Add a helper to initialise crypto stack requests and use it for
ahash and acomp.  Make sure that the flags field is initialised
fully in the helper to silence false-positive warnings from the
compiler.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202504250751.mdy28Ibr-lkp@intel.com/


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