KVM: x86/tdp_mmu: Sprinkle __must_check

The TDP MMU function __tdp_mmu_set_spte_atomic uses a cmpxchg64 to replace
the SPTE value and returns -EBUSY on failure.  The caller must check the
return value and retry.  Add __must_check to it, as well as to two more
functions that forward the return value of __tdp_mmu_set_spte_atomic to
their caller.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Message-Id: <8f7d5a1b241bf5351eaab828d1a1efe5c17699ca.1705965635.git.isaku.yamahata@intel.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>