KVM: arm64: Prevent the host from using an smc with imm16 != 0

KVM: arm64: Prevent the host from using an smc with imm16 != 0

The ARM Service Calling Convention (SMCCC) specifies that the function
identifier and parameters should be passed in registers, leaving the
16-bit immediate field un-handled in pKVM when an SMC instruction is
trapped.
Since the HVC is a private interface between EL2 and the host,
enforce the host kernel running under pKVM to use an immediate value
of 0 only when using SMCs to make it clear for non-compliant software
talking to Trustzone that we only use SMCCC.

Signed-off-by: Sebastian Ene <sebastianene@google.com>
Reviewed-by: Vincent Donnefort <vdonnefort@google.com>
Link: https://patch.msgid.link/20260330105441.3226904-1-sebastianene@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>