KVM: MMU: inline set_spte in mmu_set_spte

Since the two callers of set_spte do different things with the results,
inlining it actually makes the code simpler to reason about.  For example,
mmu_set_spte looks quite like tdp_mmu_map_handle_target_level, but the
similarity is hidden by set_spte.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>