KVM: TDX: Do not retry locally when the retry is caused by invalid memslot

KVM: TDX: Do not retry locally when the retry is caused by invalid memslot

Avoid local retries within the TDX EPT violation handler if a retry is
triggered by faulting in an invalid memslot, indicating that the memslot
is undergoing a removal process.  Faulting in a GPA from an invalid
memslot will never succeed, and holding SRCU prevents memslot deletion
from succeeding, i.e. retrying when the memslot is being actively deleted
will lead to (breakable) deadlock.

Opportunistically export kvm_vcpu_gfn_to_memslot() to allow for a per-vCPU
lookup (which, strictly speaking, is unnecessary since TDX doesn't support
SMM, but aligns the TDX code with the MMU code).

Fixes: b0327bb2e7e0 ("KVM: TDX: Retry locally in TDX EPT violation handler on RET_PF_RETRY")
Reported-by: Reinette Chatre <reinette.chatre@intel.com>
Closes: https://lore.kernel.org/all/20250519023737.30360-1-yan.y.zhao@intel.com
[Yan: Wrote patch log, comment, fixed a minor error, function export]
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://lore.kernel.org/r/20250822070523.26495-1-yan.y.zhao@intel.com
[sean: massage changelog, relocate and tweak comment]
Signed-off-by: Sean Christopherson <seanjc@google.com>