crypto: ccp: Use the stack for small SEV command buffers

[ Upstream commit e4a9af79 ]

For commands with small input/output buffers, use the local stack to
"allocate" the structures used to communicate with the PSP.   Now that
__sev_do_cmd_locked() gracefully handles vmalloc'd buffers, there's no
reason to avoid using the stack, e.g. CONFIG_VMAP_STACK=y will just work.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210406224952.4177376-6-seanjc@google.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Stable-dep-of: 91dfd982 ("crypto: ccp - Avoid page allocation failure warning for SEV_GET_ID2")
Signed-off-by: Sasha Levin <sashal@kernel.org>