KVM: SEV: Disallow pinning more pages than exist in the system

KVM: SEV: Disallow pinning more pages than exist in the system

Explicitly disallow pinning more pages for an SEV VM than exist in the
system to defend against absurd userspace requests without relying on
somewhat arbitrary kernel functionality to prevent truly stupid KVM
behavior.  E.g. even with the INT_MAX check, userspace can request that
KVM pin nearly 8TiB of memory, regardless of how much RAM exists in the
system.

Opportunistically rename "locked" to a more descriptive "total_npages".

Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Tested-by: Liam Merwick <liam.merwick@oracle.com>
Link: https://patch.msgid.link/20260313003302.3136111-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>