x86/virt/seamldr: Shut down the current TDX module

The first step of TDX module updates is shutting down the current TDX
module. This step also packs state information that needs to be
preserved across updates, called "handoff data". This handoff data is
consumed by the updated module and stored internally in the SEAM range and
hidden from the kernel.

Since the handoff data layout may change between modules, the handoff
data is versioned. Each module has a native handoff version and
provides backward support for several older versions.

The complete handoff versioning protocol is complex as it supports both
module upgrades and downgrades. See details in "Intel Trust Domain
Extensions (Intel TDX) Module Base Architecture Specification", Chapter
"Handoff Versioning".

Ideally, the kernel needs to retrieve the handoff versions supported by
the current module and the new module and select a version supported by
both. But since this implementation only supports module upgrades, simply
request handoff data from the current module using its highest supported
version. That is sufficient for this upgrade-only implementation.

Retrieve the module's handoff version from TDX global metadata and add an
update step to shut down the module. Module shutdown only needs to run on
one CPU.

Don't cache the handoff information in tdx_sysinfo. It is used only for
module shutdown, and is present only when the TDX module supports updates.
Caching it in get_tdx_sys_info() would require extra update-support guards
and refreshing the cached value across module updates.

[ dhansen: fix up function variables, remove 'cpu'.
 	   Return from tdx_module_shutdown() early if handoff call fails. ]

Signed-off-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Reviewed-by: Xu Yilun <yilun.xu@linux.intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Link: https://patch.msgid.link/20260520133909.409394-17-chao.gao@intel.com

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