KVM: x86/mmu: Return a u64 (the old SPTE) from mmu_spte_clear_track_bits()

Return a u64, not an int, from mmu_spte_clear_track_bits().  The return
value is the old SPTE value, which is very much a 64-bit value.  The sole
caller that consumes the return value, drop_spte(), already uses a u64.
The only reason that truncating the SPTE value is not problematic is
because drop_spte() only queries the shadow-present bit, which is in the
lower 32 bits.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220715224226.3749507-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>