KVM: MMU: optimize for set_spte

There are two cases we need to adjust page size in set_spte:
1): the one is other vcpu creates new sp in the window between mapping_level()
    and acquiring mmu-lock.
2): the another case is the new sp is created by itself (page-fault path) when
    guest uses the target gfn as its page table.

In current code, set_spte drop the spte and emulate the access for these case,
it works not good:
- for the case 1, it may destroy the mapping established by other vcpu, and
  do expensive instruction emulation.
- for the case 2, it may emulate the access even if the guest is accessing
  the page which not used as page table. There is a example, 0~2M is used as
  huge page in guest, in this huge page, only page 3 used as page table, then
  guest read/writes on other pages can cause instruction emulation.

Both of these cases can be fixed by allowing guest to retry the access, it
will refault, then we can establish the mapping by using small page

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Acked-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>