crypto: testmgr - don't use stack buffer in test_acomp()

With virtually-mapped stacks (CONFIG_VMAP_STACK=y), using the
scatterlist crypto API with stack buffers is not allowed, and with
appropriate debugging options will cause the
'BUG_ON(!virt_addr_valid(buf));' in sg_set_buf() to be triggered.
Use a heap buffer instead.

Fixes: d7db7a88 ("crypto: acomp - update testmgr with support for acomp")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>