KVM: x86/mmu: Fold all of make_spte()'s writable handling into one if-else

KVM: x86/mmu: Fold all of make_spte()'s writable handling into one if-else

Now that make_spte() no longer uses a funky goto to bail out for a special
case of its unsync handling, combine all of the unsync vs. writable logic
into a single if-else statement.

No functional change intended.

Link: https://lore.kernel.org/r/20241011021051.1557902-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>