KVM: MMU: unalias gfn before sp->gfns[] comparison in sync_page

sp->gfns[] contain unaliased gfns, but gpte might contain pointer
to aliased region.

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>