KVM: x86/mmu: Fix return value in tdp_mmu_map_handle_target_level()

Currently tdp_mmu_map_handle_target_level() returns 0, which is
RET_PF_RETRY, when page fault is actually fixed.  This makes
kvm_tdp_mmu_map() also return RET_PF_RETRY in this case, instead of
RET_PF_FIXED.  Fix by initializing ret to RET_PF_FIXED.

Note that kvm_mmu_page_fault() resumes guest on both RET_PF_RETRY and
RET_PF_FIXED, which means in practice returning the two won't make
difference, so this fix alone won't be necessary for stable tree.

Fixes: bb18842e ("kvm: x86/mmu: Add TDP MMU PF handler")
Reviewed-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: Kai Huang <kai.huang@intel.com>
Message-Id: <f9e8956223a586cd28c090879a8ff40f5eb6d609.1623717884.git.kai.huang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>