KVM: SVM: Allocate SEV command structures on local stack

Use the local stack to "allocate" the structures used to communicate with
the PSP.  The largest struct used by KVM, sev_data_launch_secret, clocks
in at 52 bytes, well within the realm of reasonable stack usage.  The
smallest structs are a mere 4 bytes, i.e. the pointer for the allocation
is larger than the allocation itself.

Now that the PSP driver plays nice with vmalloc pointers, putting the
data on a virtually mapped stack (CONFIG_VMAP_STACK=y) will not cause
explosions.

Cc: Brijesh Singh <[email protected]>
Cc: Tom Lendacky <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Brijesh Singh <[email protected]>
Acked-by: Tom Lendacky <[email protected]>
[Apply same treatment to PSP migration commands. - Paolo]
Signed-off-by: Paolo Bonzini <[email protected]>

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