kvm: x86: Do not release the page inside mmu_set_spte()

Release the page at the call-site where it was originally acquired.
This makes the exit code cleaner for most call sites, since they
do not need to duplicate code between success and the failure
label.

Signed-off-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>