KVM: MMU: Avoid calling gfn_to_page() in mmu_set_spte()

Since gfn_to_page() is a sleeping function, and we want to make the core mmu
spinlocked, we need to pass the page from the walker context (which can sleep)
to the shadow context (which cannot).

[marcelo: avoid recursive locking of mmap_sem]

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

diff --cc arch/x86/kvm/mmu.c
Simple merge

diff --cc arch/x86/kvm/paging_tmpl.h
Simple merge

diff --cc include/asm-x86/kvm_host.h
Simple merge