KVM: arm64: Zero out the stack initialized data in the FFA handler

KVM: arm64: Zero out the stack initialized data in the FFA handler

Don't leak hypervisor stack data when using the FFA_VERSION call.
When the compiler doesn't support -ftrivial-auto-var-init=zero option
we need to zero out the stack initialized variable before returning data
to the host caller.

Closes:
https://lore.kernel.org/all/20260616160016.C62C81F000E9@smtp.kernel.org/

Reported-by: Sashiko AI <sashiko-bot@kernel.org>
Fixes: c9c012625e12 ("KVM: arm64: Trap FFA_VERSION host call in pKVM")
Reviewed-by: Vincent Donnefort <vdonnefort@google.com>
Link: https://lore.kernel.org/all/20260616160016.C62C81F000E9@smtp.kernel.org/
Signed-off-by: Sebastian Ene <sebastianene@google.com>
Link: https://patch.msgid.link/20260702103848.1647249-7-sebastianene@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>