KVM: fast page fault

Changlog:
This are some changes from Marcelo's review:
- drop SPTE_WRITE_PROTECT bit, now, only one bit is needed to do
  lockless update.

- always atomicly update spte if it can be updated out of mmu-lock.

- flod the judgement of tlb flush into mmu_spte_update(), make the
  code easilyer audited.

Performance result:
- autoest migration test (smp2.Fedora.16.64.migrate.with_autotest.dbench.unix):
                                                   before     after
smp2.Fedora.16.64.migrate.unix                        93      91    +2.1%
smp2.Fedora.16.64.migrate.with_autotest.dbench.unix   218     188   +13.7%

- the benchmark attached is used to measure the resuming time
  after dirty-log
                            before            after
Run 10 times, Avg time:   512466818 ns.      269231261 ns   +47.5%