KVM: MMU: Fix memory leak on guest demand faults

While backporting 72dc67a6, a gfn_to_page()
call was duplicated instead of moved (due to an unrelated patch not being
present in mainline).  This caused a page reference leak, resulting in a
fairly massive memory leak.

Fix by removing the extraneous gfn_to_page() call.

Signed-off-by: Avi Kivity <avi@qumranet.com>