KVM: x86: Harden KVM against imbalanced load/put of guest FPU state

Assert, via KVM_BUG_ON(), that guest FPU state isn't/is in use when
loading/putting the FPU to help detect KVM bugs without needing an assist
from KASAN.  If an imbalanced load/put is detected, skip the redundant
load/put to avoid clobbering guest state and/or crashing the host.

Note, kvm_access_xstate_msr() already provides a similar assertion.

Reviewed-by: Yao Yuan <yaoyuan@linux.alibaba.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Link: https://patch.msgid.link/20251030185802.3375059-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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